Hooded youths burned bins, broke windows and hurled bottles and stones when police charged them after some 2,000 people gathered for a protest on Wednesday 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.
Police officials told AFP on Thursday about 30 people had been arrested. An emergency services official added that 14 people were injured.
Regional government spokesman Francesc Homs blamed the violence on well-organised radicals on the fringes of the protest.
"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.
The transport authority wants to knock it down to redevelop the area, where a rail and metro line intersect.
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.
"These protests are not just about the eviction. The eviction was just one more blow after the crisis," Guerra told AFP on Wednesday.
"The people are fed up of taking blows and in the end they explode."
The colonial-style house Can Vies was built in 1897. Sants was an independent district until that year when it was annexed by the Catalonia region.
It has kept its local, community-based character and is a "combative neighbourhood", Guerra said.
Local residents showed support for the squatters by leaning out of their windows and banging pots and pans during Wednesday's protest.
One local woman Olga Alcaraz, 45, complained of the heavy police presence in the recent nights of protest.
"That type of deployment creates more anger among the locals who are already unhappy at the situation," she said on Wednesday, referring to the first two nights of clashes.
Homs defended the police response.
"When there is an outbreak of violence like this we have to be very firm," he said.