Hundreds of thousands of Catalan independence supporters have flooded the streets of Barcelona calling for the release of jailed separatist leaders after a supreme court ruling frustrated their latest attempt to elect a regional leader.

Around 350,000 demonstrators clogged several main city arteries waving flags and wearing yellow in support of separatist leaders jailed for their role in the region's banned drive to split from Spain last year.

Among the politicians facing charges of rebellion is Jordi Sanchez, the latest candidate put forward by Catalan politicians to become leader of the region.

Supreme court judge Pablo Llarena refused last week to release him from jail and he faces up to 25 years on charges of rebellion.

It marked the latest of four unsuccessful attempts to elect a new leader.

If a new leader is not named before the end of May, Catalonia will be forced to call another election.

Many protesters claim Spain is violating the rule of law by imprisoning politicians who they believe are political prisoners.

Spain claims the Catalan leaders are not political prisoners because they broke the constitution when they held the banned referendum.

Former Catalonia president Artur Mas, the Catalan Parliament speaker Roger Torrent and members of Junts Per Catalunya Eduard Pujol and Elsa Artadi joined the demonstration.

The protest was organised by pro-independence associations ANC and Omnium Cultural whose leaders were jailed in October 2017.

In his first interview since a German court ruled that he would not be extradited to Spain to face charges of rebellion, former Catalan leader Carles Puigdemont said that the region should avoid returning to the polls.

"It's our obligation to look for a way to avoid further elections," Mr Puigdemont said in an interview with Catalan television.

"But there is not a zero risk of this happening because there is another party involved, and that's the Spanish government," he added.