Amid the Catalan independence crisis, tensions on the streets have escalated with pro-independence groups organizing protests against the trial of former Catalan separatist leaders and right-wing groups claiming for a united Spain, on the other side.

In Barcelona, thousands marched to a central square on Tuesday, demanding independence and criticizing Spain's judiciary. Some carried signs with the slogan, "Self-determination is not a crime." Earlier, pro-independence activists briefly blocked highways and the entrance to the state prosecutor's office before they were cleared by the regional police without incident. In Madrid, right-wing protesters carrying national flags shouted as lawyers and three defendants who were free on bail entered the 18th-century convent that houses Spain's Supreme Court.

On the second day of a trial for a dozen Catalan separatist leaders, a Spanish public prosecutor accused Catalan separatist leaders of having tried to use "human shields" to block police during their failed secession bid in 2017. Supreme Court prosecutor Fidel Cadena rejected arguments by defense lawyers who said the trial is politically motivated, saying "anyone can have the ideas they want."

"What is penalized... is behavior 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," he said.

Twelve Catalan separatist politicians and activists face years behind bars if they are convicted of rebellion or other charges for pushing an independence referendum in October 2017. Under Spanish law, rebellion is defined as "rising up in a violent and public manner." Prosecutors point to "violent incidents" during protests orchestrated by two grassroots groups in the lead up to the referendum.

The proceedings were broadcast live on television in a display of transparency that aims to fight the separatists' attack on the court's credibility. Authorities in Spain have dismissed the notion that the trial is political and say it follows the European Union's highest standards. Proceedings were likely to last for at least three months. The verdicts, and any sentences, will be delivered months later.

Tensions between regional and central authorities peaked with the 2017 breakaway attempt but the conflict has been festering ever since. The 7.5 million residents of Catalonia remain divided by the secession question. The separatists want Spanish Prime Minister Pedro Sanchez to agree to talks on self-determination for their region, but the government argues that Spain's constitution doesn't allow it.