Riot police were engaged in a running battle with protesters outside Barcelona's airport after Spain's Supreme Court convicted 12 separatist leaders of illegally promoting the Catalonia region's independence and sentenced nine to prison.

Police fired foam bullets and used batons against thousands of protesters who converged on Josep Tarradellas Barcelona-El Prat Airport last night after a pro-independence group put out the call.

Protesters fought back by throwing objects, spraying fire extinguishers and breaking windows.

Regional emergency service SEM said 75 people were treated for injuries at the airport. Spain's airport operator, AENA, said at least 108 flights were cancelled.

Police also clashed with angry crowds late on Monday in central Barcelona. They used batons, and sounds similar to the firing projectiles were heard.

Nine of the 12 Catalan politicians and activists were found guilty of sedition and given prison sentences of nine to 13 years. Four were additionally convicted of misuse of public funds.

The other three were fined for disobedience. The court barred all of them from holding public office.

All 12 were acquitted on the more serious charge of rebellion, which implied the use of violence, brought by state prosecutors and lawyers for the far-right Spanish party Vox.

Vox leader Santaigo Abascal criticised the verdict as too light.

Caretaker prime minister Pedro Sanchez said the outcome of the four-month trial proved the 2017 secession attempt had become "a shipwreck". He urged people to "set aside extremist positions" and "embark on a new phase" for Catalonia.

He said he hoped the prison sentences would mark a turning point in the long stand-off between national authorities and separatist legislators in Barcelona, the Catalonia region's capital.

The Catalan separatist movement is going through its most difficult period in years. With a general election scheduled for November 10, its most charismatic leaders are behind bars or abroad after fleeing to avoid prosecution.

But secessionists were defiant after the Supreme Court issued its decision in Madrid, taking to the streets, halting some trains by placing burning tyres and wood on tracks, and blocking roads as well as the airport entrance.

The convicted Catalan leaders - most of whom have been jailed for almost two years - have grown into powerful symbols for the separatists. Many sympathisers wear yellow ribbons pinned to their clothes as a sign of protest.

Catalan regional president Quim Torra described the court's verdict as "an act of vengeance". He said it "will not stop us from acting on our determination to build an independent state".

Former regional president Carles Puigdemont, who fled to Belgium in October 2017 with several others when they were summoned to appear in court, said the general election is an opportunity to show "a massive response of rejection" for the court's verdict and the "dignity and firmness" of the Catalan independence movements.

He spoke in Brussels hours after a Spanish Supreme Court judge issued an international warrant for his arrest.