(CNN) Deadly clashes between protesters and police marred voting on Sunday, as Venezuelans cast ballots on a controversial measure that could mark a turning point for their country.
The election will allow President Nicolás Maduro to replace Venezuela's current legislative body -- the National Assembly -- with a new institution called the Constituent Assembly that will have the power to rewrite the constitution.
The voting follows weeks of violent street protests in which many people have been killed or injured. On Sunday the death toll rose sharply with at least six people -- including two teenagers -- killed at protests and a National Guard officer also reported dead by the Attorney General's Office.
The death toll from the unrest ongoing since early April is 125, according to a statement from the Venezuelan attorney general's office. That number does not include at least two of Sunday's deaths, in which the reasons for the killings are under investigation.
Polls were set to close at 7 p.m. ET, after officials extended voting by one hour.
Maduro, in a 2½ minute message posted to Twitter, called the vote a historic moment.
"This has been and is a successful day with great participation," he said. "... Today is a day of victory."
International reaction was harsh, with many nations, including the United States, condemning the election.
Trump is prepared to slap sanctions on Venezuela
Clashes in the streets, call for Monday protest
A key opposition figure, Henrique Capriles, governor of Miranda state, labeled the election fraudulent and called for massive protests a noon on Monday.
Speaking at a press briefing in Caracas, Capriles said voter participation Sunday was less than 15%. He claimed voter turnout was three times higher two weeks ago for an opposition-led, non-binding referendum against Maduro's proposed Constituent Assembly.
In the streets Sunday, National Guardsmen clashed with opposition protesters and police fired tear gas at crowds in Caracas. As dozens of police officers rode motorcycles through the Altamira neighborhood, a large explosion went off. Agence France-Presse video showed two officers, each with a leg on fire as comrades rushed to help them.
At one spot in the capital, opposition demonstrators set up barricades on a highway.
News broke early that one of the candidates in the election, lawyer José Félix Pineda, had been shot dead in his home on Saturday and that opposition leader Ricardo Campos died Sunday morning.
The attorney general's office tweeted that a state prosecutor is investigating Pineda's death.
"A group of people broke into the home of the victim in the Brisas del Sur sector and shot him multiple times," the attorney general's office said in a tweet. Pineda is listed as candidate number 3 in Bolivar state.
A state prosecutor also is investigating the death of Campos, youth secretary for Accion Democratica, an opposition group. He died in Cumana, a coastal town 250 miles east of Caracas. The circumstances of his death have not been disclosed.
Young Venezuelans have taken to the streets for months to protest the vote, known locally as "la constituyente," or the constituent. The Venezuelan National Guard and protesters clash nearly every day.
The polls opened at 6 a.m. ET Sunday, with nearly 380,000 troops guarding voting stations, according to a government release. Many cast their ballots in support of the government's initiative.
Elio Herrera, who voted early in a Caracas neighborhood, called the exercise "democratic and popular."
"It was a quick and simple process," Herrera said, describing the ambiance at the polling station as happy and hopeful.
Protest banned by government
Experts have said the outcome is a foregone conclusion: Maduro will be able to consolidate political power. The opposition to Maduro fears the vote will erode democracy and give the Venezuelan leader sweeping authority.
Maduro's administration has deemed any protests illegal, threatening anyone who defies the no-protest order with up to 10 years in prison.
Maduro, who said he was the first voter in Sunday's election, called casting the first vote a symbol of the independence and sovereignty of Venezuela.
He accused US President Donald Trump of trying to "prevent the people from carrying out its right to vote," and boasted that his government is seeing through the vote despite international pressure.
"The Constituent Assembly will be the space, the power of powers, the superpower that will, so to speak, recover the national spirit, find reconciliation, justice, find the truth."
Maduro's opponents control the National Assembly, holding 112 of the body's 167 seats, and have been battling with him for political power since they won a majority of seats in December 2015. Before the winners of those elections took office, Maduro stacked the country's Supreme Court with loyalists to prevent his own impeachment.
The proposed Constituent Assembly would be made up of 545 members, all nominated by Maduro's administration. Nominees include his wife, Cilia Flores, and prominent loyalists such as former Foreign Minister Delcy Rodriguez and former Vice President Diosdado Cabello.
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People queue to cast their vote to elect a Constituent Assembly in Caracas on Sunday.
The opposition hasn't submitted any candidates for the vote because it doesn't recognize the legitimacy of the election. Ultimately, the vote and the creation of Maduro's Constituent Assembly would give the President immense political power.
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Maduro says the vote will help bring peace to a polarized country, with all branches of the government falling under the political movement founded by Maduro's late mentor and predecessor, Hugo Chavez.
Critics in Venezuela and abroad argue a Maduro mandate would erode any last signs of democracy in the country. "It would give the government the opportunity to turn Venezuela into a one-party state without any of the trappings of democracy," says Eric Farnsworth, vice president of the Council of the Americas, a business association.
A costumed demonstrator in Caracas, Venezuela, protests the government of Venezuelan President Nicolas Maduro on Sunday, August 6. Unrest continued to sweep through the country after a new legislative assembly was inaugurated following a controversial election on July 30. The new assembly has wide-ranging powers and is expected to rewrite the Venezuelan constitution at Maduro's behest. Opposition supporters, who see the vote as a power grab and an erosion of democracy, boycotted and staged demonstrations against the vote.
Anti-government activists stand near a flaming barricade in Valencia on August 6.
A man is arrested in Valencia during clashes between anti-government activists and the National Guard on August 6.
Members of the Venezuelan armed forces fly over Valencia in a helicopter while citizens demonstrate in support of a group that staged a paramilitary uprising at the Paramacay military base on August 6.
Diosdado Cabello, a member of Venezuela's National Constituent Assembly, holds an image of Venezuela's late President Hugo Chavez as delegates gather for a group photo following their swearing-in ceremony on Friday, August 4.
Opposition activists protest against the newly inaugurated National Constituent Assembly on August 4.
Opposition lawmaker Juan Requesens addresses a rally in Caracas on July 31. Two other leading opposition figures, Leopoldo Lopez and Antonio Ledezma, were rounded up from their homes, according to their families.
Maduro celebrates the results of a national vote on Sunday, July 30. His opponents boycotted the election and demonstrated against it for weeks, saying he orchestrated it to get around the existing National Assembly, which the opposition has controlled since 2015. Maduro has argued that the Constituent Assembly will help bring peace to a polarized country, with all branches of the government falling under the political movement founded by his late mentor and predecessor, Hugo Chavez.
Members of Venezuela's national police are caught in an explosion as they ride motorcycles near Altamira Square in Caracas on July 30. Venezuela has seen widespread unrest since March 29, when the Supreme Court dissolved Parliament and transferred all legislative powers to itself. The decision was later reversed, but protests have continued across the country, which is also in the midst of an economic crisis.
A wounded anti-government demonstrator is helped by medics during clashes with police in Caracas on July 30.
A demonstrator shouts slogans through a traffic cone during an anti-government protest in Caracas on July 30.
A protester wounded by a pellet gun receives attention July 30 during a demonstration against the vote for a Constituent Assembly.
A member of the National Guard fires at protesters during clashes in Caracas on Friday, July 28.
A demonstrator dressed as Venezuelan independence hero Simon Bolivar is silhouetted against a national flag in Caracas on Monday, July 24.
Venezuelan lawmakers Luis Stefanelli, left, and Jose Regnault appear stunned in a corridor of the National Assembly after a clash with demonstrators in Caracas on Wednesday, July 5. Supporters of Maduro stormed the building and attacked opposition lawmakers, witnesses said. At least seven legislative employees and five lawmakers were injured, according to National Assembly President Julio Borges. Journalists said they were also assaulted.
National Guard members clash with deputies of the National Assembly in the Parliament's courtyard in Caracas on Tuesday, June 27.
A boy runs under a national flag during a June 27 protest in Caracas.
Protester David Jose Vallenilla is shot through a fence by a member of the National Guard near a military base in Caracas on Thursday, June 22. Vallenilla later died in the hospital after suffering three gunshot wounds to the chest.
Maduro holds up a copy of the Venezuelan constitution during a news conference at the presidential palace in Caracas on June 22. Maduro has called for changes to the constitution amid the unrest.
A protester hides behind a barrier in Caracas on June 22.
An opposition activist displays bullet shells during an anti-government demonstration on Monday, June 19.
People flee during a clash between opposition demonstrators and riot police in Caracas on June 19.
Opposition activists aim projectiles toward riot police during a demonstration on June 19.
Demonstrators stand in front of a police vehicle on June 19.
A street in eastern Caracas is painted with the names of people killed during more than two months of protests.
During a "Grandparents' March" in Caracas, a man is blocked by police from reaching the Government Ombudsman's Office on Friday, May 12.
Opposition activists scuffle with riot police in Caracas on May 12.
Thousands of Venezuelan protesters march on a Caracas highway on Wednesday, May 10.
Opposition activists clash with riot police in Caracas on Monday, May 8.
An armored National Guard vehicle runs over a protester in Caracas on Wednesday, May 3. The protester, 22-year-old Pedro Michell Yaminne, survived, his mother told CNN. Interior and justice minister Nestor Reverol told reporters that the "lamentable" incident was under investigation. He said that moments before Yaminne was run over, demonstrators hurled a Molotov cocktail at the armored vehicle, opened the side door and "brutally assaulted" the driver.
A demonstrator catches fire during protests in Caracas on May 3. It happened as protesters clashed with police and the gas tank of a police motorcycle exploded. Other photos from the scene showed the man being attended for burns to his body.
A demonstrator stands in front of an armored vehicle during protests in Caracas on Wednesday, April 19.
Opposition leader Henrique Capriles reacts to tear gas during a protest on April 19.
Riot police clash with demonstrators while tear gas fills the air in Caracas on April 19.
Rescue workers tend to a demonstrator hit by a tear gas canister during anti-government protests on April 19.
A demonstrator reacts during a march in Caracas on Saturday, April 15.
A protester suffering from the effects of tear gas is carried away on Thursday, April 13.
Clouds of tear gas spread across a highway in Caracas during clashes on Monday, April 10.
Demonstrators help a journalist whose leg was injured while covering clashes in Caracas on April 10.
Thousands of demonstrators protest against Maduro in Caracas on Saturday, April 8.
Venezuelan police line up before clashing with opposition activists on Thursday, April 6.
Demonstrators fight with national police officers in Caracas on Tuesday, April 4.
Trump recently said in a statement that Maduro "dreams of being a dictator." His administration sanctioned 13 Venezuelan leaders associated with Maduro this week.
On Sunday, the US State Department issued a statement condemning the vote: "The United States stands by the people of Venezuela, and their constitutional representatives, in their quest to restore their country to a full and prosperous democracy. We will continue to take strong and swift actions against the architects of authoritarianism in Venezuela, including those who participate in the National Constituent Assembly as a result of today's flawed election."
Mexico, Colombia and Panama followed with sanctions of their own against the same individuals. Argentina and Mexico said they won't recognize Sunday's vote results. Canada denounced the vote.
The Organization of American States deems the vote illegal. Spain said it will study, along with its partners in the European Union and countries friendly to the region, additional measures that may be effective in promoting a restoration of democratic institutions.
Nicaragua came to Venezuela's defense, praising it for the "historic day for the brave people of Bolívar y Chávez, the President Nicolás, the United Socialist Party."
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Trump warned he would take further "strong and swift economic action" against Maduro after the vote, though his administration hasn't elaborated beyond saying all options are on the table.
Venezuela's ministry of communication did not respond to CNN's request for comment on the vote.
Related: Trump sanctions 13 Venezuelans tied to Maduro
These are the faces of #laresistencia. Most are college educated, and some speak fluent English. All say they want to return to a normal life. #caracas #venezuela
On July 16, more than 7 million Venezuelans cast ballots in an unofficial vote against Maduro's Constituent Assembly. Maduro ignored the results of the vote, which was organized by opposition leaders.
CNN's Mariano Castillo, Natalie Gallón and Julia Jones contributed to this report