* Peaceful protesters toss balls, skip rope
* Catalan regional chief urges parties to back secession
* Dozens injured in overnight clashes in Barcelona (Rewrites with protesters assembling)
By Marco Trujillo, Jordi Rubio and Joan Faus
BARCELONA, Oct 17 (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 prison sentences for their roles in a 2017 bid for independence.
Protesters draped in the Catalan flag gathered peacefully on Thursday, some tossing balls and skipping rope. Nearly 100 people were injured across the northeastern region of Catalonia on Wednesday while cars and garbage bins were torched overnight.
Earlier, thousands of students took to the streets, some of them hurling eggs at police holding riot shields.
"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.
Independence is a highly divisive issue that polarises the wealthy region and inflames political sentiment in the rest of Spain. Neither those wanting a split from Spain nor those favouring unity have a majority in the region.
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.
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.
Addressing the Catalan parliament, pro-independence regional chief Quim Torra criticised 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 totalling 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, 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. (Reporting by Marco Trujillo and Jordi Rubio; additional reporting by Joan Faus, Guillermo Martinez in Barcelona, Ashifa Kassam, Emma Pinedo, Clara-Laeila Laudette, Isla Binnie, Jose Elías Rodríguez and Ingrid Melander in Madrid, Writing by Crispian Balmer and Sonya Dowsett; Editing by Alison Williams and Grant McCool)