BARCELONA (Reuters) - Thousands gathered in Barcelona in a fourth day of protests that have led to the worst sustained street violence in Spain in decades after Catalan leaders were sentenced to lengthy jail terms for their roles in a 2017 bid for independence.

Young people draped in Catalan flags congregated peacefully, tossing balls and skipping rope. Later the mood turned ugly, with protesters setting fire to cafe chairs on the fashionable Rambla de Catalunya street at the heart of the tourist district.

Earlier in the day, thousands of students took to the streets, some hurling eggs at police holding riot shields. Marches from around the region are due to converge on Barcelona on Friday and unions have called a general strike for the day.

“It’s not about who is a separatist and who is not - it’s about human rights,” said Aila, a student who declined to give her family name.

Spain’s Supreme Court found nine politicians and activists guilty of sedition on Monday and sentenced them to up to 13 years in jail.

Thousands barricaded the airport on Monday, leading to canceled flights and stranded tourists. On Wednesday, nearly 100 people were injured across the region in unrest while cars and garbage bins were torched overnight.

Independence is a highly divisive issue that polarizes the wealthy region and inflames political sentiment in the rest of Spain. Neither those wanting a split from Spain nor those favoring unity make up a majority of the region’s population.

Pro-independence leaders staged their banned referendum in 2017 and then declared they were breaking away from Spain. The Spanish government immediately seized control of the Catalan administration and the ringleaders were put on trial.

The riots come at a crucial time for the acting prime minister, Socialist Pedro Sanchez, as he prepares for the country’s fourth election in as many years on Nov. 10.

Slideshow ( 45 images )

The right-wing opposition has called for a more forceful crackdown on the pro-independence regional government.

Catalonia should hold a new vote on independence from Spain within two years, the head of the region’s government said on Thursday in a fresh challenge to Madrid.

Slideshow ( 45 images )

Addressing the Catalan parliament, pro-independence regional chief Quim Torra criticized the rioting, saying the separatist cause was a peaceful movement.

But he also pushed the case for forging ahead with the stalled secessionist drive, saying the prison sentences for separatists would not deter a new vote on independence.

“We’ll return to the ballot box again on self-determination,” he said.

Barcelona town hall said 400 garbage containers were set ablaze on Wednesday and estimated that the city had suffered damage totaling more than 1 million euros ($1.1 million) in two days. Some city residents condemned the rioting.

“This doesn’t represent the majority of Catalans, whichever side they are on, be they pro-constitution or pro-independence,” said Joan, a 50-year-old small-business owner.

Carmaker Seat, a unit of Volkswagen AG VOWG_p.DE has halted production at its plant in Martorell, near Barcelona, from Thursday afternoon until Saturday, as it expects planned marches to disrupt traffic, a company spokeswoman said.