The 12 defendants — including former Catalan leader Carles Puigdemont currently in self-imposed exile — face up to 25 years in jail.
Spain's Supreme Court sentenced nine Catalan separatist leaders to between 9 and 13 years in prison for sedition in their role in the region's 2017 failed bid for independence.
The three other defendants were found guilty only of disobedience and not sentenced to prison. All defendants were acquitted of the most severe charge — rebellion.
The charges all relate to the independence referendum held in Catalonia on October 1, 2017. The vote was declared illegal by the Spanish government, which moved immediately to punish those involved.
The former head of Catalonia's regional government Carles Puigdemont classified the prison sentences as an "atrocity".
"It is time to react ... for the future of our sons and daughters. For democracy. For Europe. For Catalonia," Puigdemont said on Twitter.
Puigdemont was not part of the trial because he fled to Belgium where he lives in self-imposed exile.
The former deputy leader of Catalonia and head of the pro-independence Esquerra Republicana party Oriol Junqueras received the harshest sentence: 13 years for sedition and misuse of public funds.
The court convinced Junqueras and eight other leaders on charges of sedition and four of them of misuse of public funds, the court ruling showed.
In a letter published on his party's Facebook page, the independentist leader said the movement "would return stronger and to win."
The jailed Catalan leader also told Euronews in an interview earlier this year that he would never give up for self-determination for the Catalan region of Spain.
The Spanish government launches a countercampaign: #ThisIsTheRealSpain:
At the same time that the sentence was read, the Government of Spain and its main leaders launched a campaign on social media under the hashtag #Everybodysland.
"Spain is a consolidated democracy. It's one of the most advanced Rules of Law in the world and one of the freest and safe countries", Prime Minister Pedro Sánchez claimed on Twitter.
The video depicts popular figures of the socialist Spanish Government**,** such as Josep Borrell, designated candidate for High Representative for foreign affairs in the European Union, acting Minister of Economy Nadia Calviño or Spanish astronaut and acting Minister of Science Pedro Duque.
Spain's Prime Minister Pedro Sánchez also appeared in Moncloa earlier this morning and expressed his absolute respect for and compliance with the Supreme Court's ruling.
The verdict of a historic trial
The high-profile trial started in February and saw more than 300 witnesses testify. When it ended in mid-June, Ignacio González Vega, the spokesman for the professional association for Spanish Judges for Democracy, said that "the nuclear question is whether or not there has been violence that integrates the crime of rebellion".
He believes that “accusations are still based on the fact that (the declaration of independence on October 1) was a planned plot spread out through time with episodes of violence while the defence still argues that the declaration of independence was made without any violence.”
Some Catalan separatists have, however, accused the country's courts of being heavily influenced by politicians.
If the Supreme Court rules against them, the defendants will be able to appeal to the Constitutional Court and then the European Court of Human Rights in Strasbourg.
What might happen now is largely unknown. Separatist protests have been largely peaceful but police told Reuters authorities are prepared for any outbursts of violence.
After the ruling, the CDR grassroots movement tweeted: "It's time to rise up against the authoritarian fascism of the Spanish state and its accomplices. It is time for the #RevoltaPopular (popular revolt)."
On Saturday, around 10,000 people marched in Madrid on the country's National Day to protest Catalan independence.