Preliminary results show that 90% of those who voted in Catalonia's independence referendum backed leaving Spain, Catalan officials say.
Catalonia's government said that of the 2.2m voters who were able to cast ballots in the banned referendum, two million voted for independence.
:: Catalonia referendum: The fight for independence explained
Regional president Carles Puigdemont said Catalonia had "won the right to become an independent state", adding he would keep to his vow to declare independence unilaterally if separatists win more than 50% of the vote.
However, Spanish PM Mariano Rajoy insisted that no independence referendum had occurred and that those who took part had been "fooled" into participating in a vote declared illegal by the country's constitutional court.
There were violent clashes as security forces dispatched by Madrid blocked polling stations and forcibly seized ballot boxes in an attempt to disrupt the vote.
Catalan officials claimed 844 people were injured, while Sky's Europe Correspondent Mark Stone said he had seen voters pulled out of polling stations by their hair and stamped on by police.
Rubber bullets - illegal in Catalonia - were also used to disperse crowds, he added.
Challenged by Stone about the tactics used, Spain's foreign minister Alfonso Dastis said: "I don't agree with you that this is an extraordinary level of violence.
"You may think people were peacefully exercising their right to vote but the problem is this so-called referendum had been held to be illegal by the constitutional court."
During the clashes, riot police stormed a polling station near Girona, to the north of Barcelona, using a hammer to smash through a glass door of a school.
Mr Puigdemont had been due to appear at the polling station and was forced to cast his ballot in nearby Cornella del Terri instead.
In a televised address, Mr Rajoy thanked police for their clampdown on the vote and said they had acted with "firmness and serenity".
Accusing separatists of attempting to "blackmail" Spain, the PM offered all-party talks on the region's future.
Meanwhile, unions and workers' associations have called for a general strike to be held across the region on Tuesday, urging Catalans to protest against a "grave violation of rights and freedoms".
Clashes between voters and police were condemned by UK politicians, including Labour leader Jeremy Corbyn, who tweeted: "I urge @theresa_may to appeal directly to Rajoy to end police violence in Catalonia & find political solution to this constitutional crisis."
But the British Foreign Office gave tentative backing to Madrid.
A spokesperson said: "The referendum is a matter for the Spanish government and people. We want to see Spanish law and the Spanish constitution respected and the rule of law upheld.
"Spain is a close ally and a good friend, whose strength and unity matters to us."