(CNN) Spain is facing political turmoil this week following a contested independence referendum in the region of Catalonia, which descended into violence and left hundreds injured.
Here's what you need to know.
More than 2.25 million people turned out to Sunday's referendum across Catalonia, a region in the northeast of Spain. The regional government said 90% of voters were in favor of a split from Madrid.
But the turnout was low -- around 42% of the voter roll. Catalan authorities blamed the figure on the crackdown on the vote initiated by the national government.
Spain's highest court had ruled the vote illegal under the Spanish constitution. Citing the judicial authority, Madrid flooded Catalonia with thousands of national police in advance of the vote. Officers seized millions of ballot papers and sealed schools and other buildings to be used as polling stations.
On Sunday, the day of the disputed vote, national police launched a concerted effort to prevent people from casting their ballots. Police fired rubber bullets at protesters and voters trying to take part in the referendum, and used batons to beat them back.
Police smashed their way into polling stations, and were seen pulling voters out by the hair and restraining elderly people.
The scenes shocked Catalans and reverberated around Europe.
Almost 900 people were injured, Catalan officials said. Opposition parties criticized Spanish Prime Minister Mariano Rajoy for taking a heavy-handed approach to blocking the vote.
The long-running dispute goes back to the brutal years under Franco, whose dictatorial regime repressed Catalonia's earlier limited autonomy. In 1979, four years after his death, the region was granted greater autonomy.
In 2006, the Spanish government backed Catalonia's calls for even greater powers and financial control of the region, granting it "nation" status.
But four years after that, the status was rescinded by the Constitutional Court, which ruled that while Catalan is a "nationality," Catalonia is not itself a nation.
Catalonia's campaign to break away has been gaining momentum since 2010, when Spain's economy plunged during the financial crisis. Catalonia is the wealthiest region in Spain and is industrially more advanced than the rest of the country.
READ: Spain loses 20% of its economy if Catalonia splits
The region held a symbolic poll in 2014, in which 80% of voters backed complete secession -- but only 32% of the electorate turned out.
Madrid insists the vote was illegal. Prime Minister Rajoy said on Sunday that it "didn't happen."
The national government argues that Spain's Constitution, revised in 1978, says that all Spaniards have the right to vote in referenda on major issues, so any vote on secession must be put to all Spaniards. Less than half of the regions 5 million voters appear to have taken part in the referendum.
The European Commission wrote on Twitter that the vote was not legal.
There are several other complications with the vote. Because Madrid declared the vote illegal, many Catalans who might have turned out to an independence referendum chose not to.
Images of the use of force by police also suggest that many voters were physically prevented from casting their ballots, or felt intimated to do so.
The world is now waiting to see if Catalan officials will unilaterally declare independence. Authorities had earlier threatened to make the declaration within 48 hours of the vote if secession won.
But on Monday, the Catalan President stopped short of such a declaration, calling for international mediation instead.
Madrid has the option of seizing control of Catalonia's administration by triggering article 155 of the Constitution. It could use this time to call a snap election and negotiate with new Catalan leaders.
But this is a major risk for the Spanish government. Like the use of force on Sunday, such a measure would likely fuel resentment for Madrid among Catalan independence supporters and create more political turmoil.
There is more at stake for Madrid than losing its wealthiest region. The country has 17 regions with varying degrees of autonomy, and losing one may inspire other regions to begin, or revive, separatist movements.