Backlash: Protesters clash with police as they demonstrate at the airport, after Catalan leader were jailed. Photo: REUTERS/Albert Gea

Riot police last night 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 after a pro-independence group put out the call.

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

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

Police also clashed with angry crowds late last night in central Barcelona. They used batons, and residents reported hearing foam rounds being fired.

Nine of the Catalans on trial in association with their efforts to achieve independence received between nine and 13 years in prison for sedition.

Four of them were additionally convicted for misuse of public funds, and three more were fined for disobedience.

The Spanish constitution says the country can't be divided.

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.

The landmark ruling, after a four-month trial, inflamed independence supporters in the north-eastern region bordering France where Catalan identity is a passionate issue.

Spain's caretaker prime minister, Pedro Sánchez, said he hoped the sentence would mark a watershed in the long standoff between national authorities in Madrid and separatists in the Catalan capital Barcelona.

Mr Sánchez said the court's verdict 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.

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.

Former regional president Carles Puigdemont, who fled in October 2017 when 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.

Grassroots pro-secession groups previously had warned that guilty verdicts would bring what they called "peaceful civil disobedience".

Spanish authorities deployed hundreds of extra police to the region in anticipation of the ruling.

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".

In their ruling, the seven Supreme Court judges wrote that what the Catalan leaders presented as a legitimate exercise of the right to decide was in fact "bait" to mobilise citizens and place pressure on the Spanish government to grant a referendum on independence.

Irish Independent