An exceptional police presence was in place in Barcelona on Saturday ahead of next week's key ECB meeting, with authorities fearing violence in Spain's climate of social unrest.
It is "one of the biggest ever deployments by the Mossos d'Esquadra, the Catalan regional police, with 4,500 police mobilised until May 4, a spokesman told AFP, saying that security was on the same level as for the visit by Pope Benedict XVI in November 2010.
They may also be reinforced by 2,000 members of the Civil Guard, the interior ministry said.
Authorities fear fresh violence as the city hosts a meeting of the European Central Bank's governing council beginning Thursday to discuss monetary policy at a time when Spain is a major focus of concern.
With unemployment running at a record 24.4 percent of the work force and the country officially back in recession, pressure for Spain to stabilise its public finances is driving spending cuts and unpopular austerity measures that are fuelling social unrest.
Trade unions have called a nationwide demonstration Sunday to protest against the austerity cuts ahead of a Labour Day march on Tuesday and a mass demonstration by students against raising university fees.
Spain, a member of Europe's free-movement Schengen treaty, has also temporarily restored border checkpoints with France this month ahead of the ECB meeting.
"Reports by the state security forces have detected the possibility that anti-establishment organisations or movements could come for this meeting," an interior ministry official told AFP.
"This measure has been adopted as a precaution in case of disturbances by any anti-establishment group," the official added. "We are talking about acts of violence."