Hooded youths burned bins, broke windows and hurled bottles and stones when police charged them after some 2,000 people gathered for a protest last night, an AFP reporter saw.
The crowd had massed near Can Vies, a public building facing demolition after being occupied for 17 years by leftist groups in the northeastern Spanish city.
Helmeted police charged at protesters after some breached a security line, an AFP reporter saw. Rioters then tipped over and burned bins, broke windows and pelted police with missiles.
Regional government spokesman Francesc Homs blamed the violence on well-organised radicals on the fringes of the protest.
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"These people have little to do with the reasons for the demonstration. They are professional troublemakers," he said.
Police had arrested eight other people in two previous nights of clashes that erupted after squatters were cleared from Can Vies and a digger started knocking it down.
The house in the working-class Sants neighbourhood is owned by the city transport authority but was occupied in 1997 by activists who used it to host concerts, training courses and other community activities.
Police cleared the house following a court order issued after negotiations broke down between the city and the squatters.
"This is an attack on an alternative model for a neighbourhood and a city", said a Can Vies representative going by the nom de guerre Pau Guerra.
Spain has seen numerous protests against home evictions, hardship and cuts to public services due to the economic crisis that erupted in 2008.