Violent protesters clashed Saturday with police in the tense Catalonia region of Spain after another massive rally in Barcelona against the imprisonment of nine separatist leaders.

Thousands of pro-independence demonstrators - mostly masked youths - took to the streets in the first flare-up in violence after a week of tense calm. They surrounded the National Police headquarters in downtown Barcelona and threw colorful plastic balls at officers. The protesters quickly switched to lobbing rocks and bottles at police, who then charged and swung batons to fend them off.

Regional emergency services said 15 people were being treated for injuries as a result of the clashes, including an AP photographer who was hit in the face with a police baton. Catalonia's regional police said one of its officers was serious hurt. At least one person had to be put on a stretcher by medics and taken away in an ambulance.

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Earlier Saturday Barcelona’s police said 350,000 rallied peacefully in the capital, many waving pro-independence flags for Catalonia. Pro-secession grassroots groups organized the rally to call for a new state in the northeastern corner of Spain.

"We cannot accept that (the prisoners) have been condemned to terms of nine to 13 years for defending the self-determination of Catalans," the president of the pro-secession grassroots group ANC, Elisenda Paluzie, said at the peaceful protest.

Catalonia held its first vote for independence in November 2014. At the time, 80.8 percent of voters said they wanted Catalonia to be an independent state. Today Catalonia’s 7.5 million residents are divided roughly in half between those opposing and those in support of secession. Spain’s government has told Catalonia’s separatists that the national Parliament would have to amend the Constitution, which considers the nation indivisible, to make secession legal.

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Catalonia was an independent state until the early 1700s with the birth of modern-day Spain. The region was granted a degree of autonomy in 1977, but calls for a completely separate state persisted. The economic crisis in Spain which blew up in 2008 only magnified calls for Catalan independence, where the wealthy Barcelona region was seen as propping up the poorer rest of the country.

On Oct. 12, the Spanish Supreme Court found nine of the 12 Catalan separatist leaders guilty of sedition and misuse of public funds for organizing an independence referendum in 2017. The nine leaders received sentences of anywhere from nine to 12 years each. The other three were fined for disobedience.

A match between Barcelona FC and rival Real Madrid was originally scheduled for Saturday, but the game was postponed due to the protests.

Alternatively, Spanish unity groups in the region have called for a rally on Sunday.

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The political turmoil is set to be a key issue in Spain’s upcoming national election, where Socialist Prime Minister Pedro Sánchez will try to maintain his seat. Spain’s far-right party Vox however is trying to convince voters it is the best option to heal divides in Spain.

The Associated Press contributed to this report.