A demonstrator holds a flare during protest after a verdict in a trial over a banned Catalonia's independence referendum in Barcelona, Spain, on October 16. Photo: VCG

Spain's acting prime minister hardened his stance on Catalonia in a live TV debate with his rivals on Monday, ahead of a repeat general election which polls suggest will lead to ­another inconclusive outcome.

Sunday's election comes ­after several days of violent protests in Barcelona and other Catalan cities, sparked by last month's jailing of nine Catalan separatist leaders for a failed 2017 independence bid.

Acting Prime Minister Pedro Sanchez vowed during the five-way debate to amend Spain's laws to make clear "once and for all" that staging an unsanctioned independence referendum would be a crime if his Socialists were re-elected.

Sanchez, who took office in June 2018 with the support of Catalan separatist lawmakers, came under fire from the right over his handling of the protests, which saw demonstrators set fire to cars and throw rocks, Molotov cocktails and steel balls at police.

Holding up a piece of pavement "like those that were thrown at police" in Barcelona, the leader of the center-right party Ciudadanos, Albert Rivera, said Madrid should have restored order by suspending Catalonia's regional autonomy and dismissing the head of Catalonia separatist regional government, Quim Torra.

The leader of far-right party Vox, Santiago Abascal, went further, saying there was a "permanent coup d'etat in Catalonia" and calling for Torra to be "arrested, handcuffed and brought before the courts."