Around 200,000 people have marched through Barcelona against the high-profile trial of Catalan separatist leaders that started this week, which they dismiss as a farce.

"Self-determination is not a crime," read a banner at the front of the protest led by regional president Quim Torra which municipal police said was attended by 200,000 people, while organisers said over 500,000 had taken part.

Demonstrators waved blue, red and yellow Catalan separatist flags and carried banners reading "Freedom for political prisoners."

The trial of 12 Catalan separatist politicians and activists over their role in the 2017 attempt to break Catalonia from Spain started on Tuesday at Madrid's Supreme Court.

They face up to 25 years behind bars on charges of rebellion and other offences for pushing an independence referendum in October 2017 in defiance of a court ban, and for a brief declaration of independence.

The independence bid sparked Spain's deepest political crisis since the transition to democracy in the 1970s after the death of dictator Francisco Franco.

Independence supporters and the defendants' lawyers maintain they are on trial for their ideas and for political dissidence.

"What's happening is really sad. It's a political trial, full of manipulations to sentence them for something that isn't a crime," said Jesus Rodriguez, head of a group of 'castellers,' Catalonia's famous tradition of giant human towers, at the Barcelona protest.

"They just want to lock them up and as they know that voting isn't a crime, they're inventing violence that never happened," he added.

Prosecutors though insist the separatist leaders and activists are being tried for their actions and not for what they think.

Many Spaniards support the trial, shocked by the actions of Catalonia's regional executive in October 2017.

Controversy has swirled over the charge of rebellion.

Under Spanish law, rebellion is defined as "rising up in a violent and public manner". But opinion is divided  over whether the independence bid was violent.

At the trial on Wednesday, prosecutor Fidel Cadena said "what is behaviour carried out over time which aimed... at the subversion and rupture of the constitutional order, calling for violent methods through the use of the masses as human shields" against police.

The trial is scheduled to last three months, with verdicts expected several months later.