Madrid (CNN) Nine Catalan independence leaders have received lengthy prison sentences of between nine and 13 years for their part in a failed 2017 attempt to split from Spain, but were acquitted of the most serious charge against them.

They were found not guilty on charges of rebellion, which carries a sentence of up to 25 years in prison.

Three other defendants were found guilty of disobedience, fined and banned from public office for 20 months.

Oriol Junqueras, the former vice president of Catalonia, received the longest sentence. He was handed 13 years in prison and banned from holding public office for 13 years after being found guilty of sedition and misuse of public funds.

In 2017, Spain descended into its worst political crisis since the restoration of democracy in the 1970s, when separatist leaders attempted to push forward with the region's secession.

Police and protestors clashed in the streets as a referendum -- ruled illegal by Spain's constitutional court -- went ahead and was followed by a declaration of independence in October that year.

Catalonia President Quim Torra described Monday's sentences as an "act of revenge, not of justice" by the Spanish government.

Torra told reporters that he had asked to appear in front of the Catalonian parliament as soon as possible and would urgently send a letter to the King of Spain and Spanish Prime Minister Pedro Sanchez.

"Catalonia lives a critical hour," Torra told reporters, describing the 12 defendants as "honorable and peaceful people."

"We demand amnesty. Threat, imprisonment and punishment are not the solution. That is the inheritance of the dictatorship, not a trait of democracy."

Protesters clash with Spanish police on the highway leading to El Prat airport in Barcelona on October 14, 2019.

Prime Minister Sanchez told reporters that the Spanish government had "absolute respect for and adherence to" the Supreme Court's ruling.

"Nobody is above the law, and we must all comply with the law," he said. "In a democracy, no one is judged for their ideas or for their political projects, but for crimes defined in Spanish law."

The Second Chamber of the Spanish Supreme Court also said on Monday that it has issued an European and international arrest warrant for Carles Puigdemont, the former head of the Catalan regional government, "for crimes of sedition and embezzlement of public funds."

Judge Pablo Llarena reissued a warrant for Puigdemont, now living in self-imposed exile in Belgium, which he had withdrawn in July 2018 after Germany had refused to hand him over.

Police clash with protesters

Pro-independence protesters flocked to Barcelona's El Prat airport Monday afternoon following the verdict, where police said officers were required to "intervene" in order to prevent them from getting inside.

Video from the scene showed police using batons on protesters outside the building, as traffic cones were hurled at officers.

Protesters gather outside El Prat airport in Barcelona on October 14.

Journalist Guillem Andres Ruiz, who was outside Terminal 1, told CNN that he estimated "thousands" of protesters tried to enter the airport's terminal.

Ruiz said he saw one person with a head injury following the clashes and that police had "established a line 100 meters from the arrivals entrance."

A police spokesman told CNN that officers fired "foam projectiles" at some protesters at the city's airport and that two people had been arrested for attacking police personnel.

According to AENA -- the airport's operator -- 108 flights were canceled.

The leaders sentenced

Former Catalan foreign minister Raul Romeva, Jordi Turull, the former Catalan government spokesman, and ex-labor minister Dolors Bassa were convicted of sedition and misuse of public funds and sentenced to 12 years in prison. They were also banned from holding public office for 12 years.

Sedition is defined in Spanish law as the act of leading "the citizenry in a public and tumultuous rising which prevents the application of law and obstructs compliance with court decisions."

Carme Forcadell, the former speaker of the Catalan parliament, was sentenced to 11 years and six months in prison, and banned from holding public office for the same period of time, for sedition.

Former Catalan interior minister Joaquim Forn and former territorial minister Josep Rull were both sentenced to 10 years and six months in prison and banned from holding public office, while influential grassroots activists Jordi Sanchez and Jordi Cuixart, were sentenced to nine years in prison and also banned from public office.

Three others -- Santiago Vila, Meritxell Borras and Carles Mundo -- were found guilty of disobedience, fined and banned from public office for 20 months.

Puigdemont described the sentences as an "outrage." He posted on Twitter: "100 years in prison in total. An outrage. Now more than ever, with you and with your families. It's time to react, like never before. For the future of our children. For democracy. For Europe. For Catalunya."