Hundreds of masked youths clashed angrily with police in Barcelona on the sidelines of a mass march which drew more than half a million people in protest over Spain's jailing of nine Catalan separatist leaders.
It was the biggest gathering since Monday's explosive verdict, which had already brought tens of thousands of independence supporters onto the streets in protest in this wealthy northeast region.
Police said 525,000 people had joined Friday's giant protest, the latest in a mass show of anger after Spain's Supreme Court sentenced nine separatists to long jail terms over a banned referendum and an abortive independence declaration two years ago.
But while most marchers appeared peaceful, hoards of young protesters went on the rampage in Via Laietana, setting a huge blaze which sent plumes of black smoke into the air, as police fired teargas to disperse them, an AFP correspondent said.
"Anti-fascist Catalonia!" they roared. "The streets will always be ours!" Elsewhere among the crowds were many thousands of "freedom marchers", who had set out from five regional towns on Wednesday to walk to the Catalan capital, many wearing walking boots and carrying hiking poles.
The rally coincided with a general strike, prompting the cancellation of 57 flights, the closure of shops, business and several top tourist attractions, and slowing public transport to a trickle in a region that accounts for about a fifth of Spain's economic output.
Activists also cut off Catalonia's main cross-border highway with France.
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In downtown Barcelona, many shops and luxury outlets were closed on the city's Paseo de Gracia, with blackened, charred patches a testimony to the nightly clashes that have raged every evening since Monday.
"With these demonstrations bringing this large city to a halt, we are using Barcelona like a microphone," said 23-year-old engineering student Ramon Parada.
"It's all in reaction to the injustice of the verdict against politicians and civil leaders who acted peacefully but were sentenced to between nine and 13 years of prison, like murderers," he said.
Retired lawyer Jaume Enrich agreed, saying the sentence was "the straw that broke the camel's back." "Madrid is putting Spanish unity above everything, including basic rights," he told AFP, wearing a badge saying "No surrender".
Nearby a banner fluttered reading "There are not enough cages for this many birds."
"We've had seven years of lies. They only tell the teenagers bad things about Spain."
"The international community must help us to solve the conflict between Catalonia and Spain," he said on Friday. "Some mediator from outside (must) help us sit (down) and talk."