(CNN) The official results of Bolivia's presidential elections are still to come, but centrist candidate Carlos Mesa has accepted defeat, all but clinching a Socialist victory.
The Socialist candidate Luis Arce is already receiving congratulatory messages.
"We will have to lead the opposition. We will make Bolivia proud," Mesa said on Twitter Monday morning. He also thanked the voters who had supported him.
The country's interim President Jeanine Añez has said the results so far suggest Arce has won.
"We still don't have the official counting, but from the data we have, Mr. Arce and [vice-presidential candidate] Mr. [David] Choquehuanca have won the election. I congratulate the winners," Añez said on Twitter.
Arce, the frontrunner in the race, represents the Movement to Socialism, the party led by former president Evo Morales who is currently exiled in Argentina. Arce, whom Morales handpicked as his successor, has previously served as the country's finance minister.
His closest competitor was Mesa, who leads Bolivia's Citizen Community party.
According to Bolivia's electoral rules, a candidate needs at least 40% of the votes to win, and a lead of 10% ahead of his opponent to win during the first round.
Less than 20% of the votes have been officially counted, and it could take days before the election's winner is formally proclaimed. Unofficial exit polls suggest Arce has a strong enough lead to claim the presidency.
Bolivia has been badly hit by coronavirus. Añez herself contracted the virus, along with roughly a dozen members of her senior cabinet.
The economy is also struggling. Unemployment has spiked since the pandemic began, the International Monetary Fund is predicting a nearly 8% drop in GDP this year, and last month, US credit ratings agency Moody's downgraded Bolivia's standing.
Meanwhile, former president Morales, who is living in Buenos Aires after fleeing Bolivia amid allegations of voting fraud last year, expressed his wish to return to Bolivia.
Speaking with CNN, Morales said: "Sooner or later we will return to Bolivia," adding that his return was "just a matter of time."