By John O'Donnell and Frank Siebelt
FRANKFURT, March 18 (Reuters) - Anti-capitalist protesters clashed with riot police near the new headquarters of the European Central Bank (ECB) in Frankfurt on Wednesday and set fire to barricades and cars, casting a pall over the ceremonial opening of the billion-euro skyscraper.
Nearly 90 police were injured by stones and unidentified liquids hurled by a violent minority from within the thousands-strong protest, police said. Some protesters said they were injured when police used pepper spray.
Seven police cars were set on fire, streets were blocked by burning stacks of tires and rubbish bins, and shops were damaged in the city center. Dark smoke billowed in front of the ECB towers and across central Frankfurt.
Police used water cannon to try to make a path through the mass of black-clad protesters to the entrance of the building, blocked off from the street by police barricades. Five people were detained and others taken into custody for questioning.
ECB President Mario Draghi addressed the demonstrators in his speech at the opening ceremony but said they were missing the point by blaming the ECB.
"European unity is being strained," he said, according to an advance text. "People are going through very difficult times. There are some, like many of the protesters outside today, who believe the problem is that Europe is doing too little.
"But the euro area is not a political union of the sort where some countries permanently pay for others. It has always been understood that countries have to be able to stand on their own two feet - that each is responsible for its own policies. The fact that some had to go through a difficult period of adjustment was therefore not a choice that was imposed on them. It was a consequence of their past decisions."
The protest organizers, a group called Blockupy - named after the Occupy Wall Street movement in 2011 - estimated that about 10,000 demonstrators were at the rally. Thousands came into the German financial capital from other parts of Europe.
"Our protest is against the ECB, as a member of the troika, that, despite the fact that it is not democratically elected, hinders the work of the Greek government. We want the austerity politics to end," Ulrich Wilken, one of the organizers, said.
"We want a loud but peaceful protest," he told Reuters.
Blockupy says it represents grass-roots critics of supranational financial institutions including the "troika": the ECB, the European Commission and the International Monetary Fund, whose inspectors monitor countries such as Greece and Cyprus that have received international bailouts.
The ECB is also influential as a provider of finance to the banks of struggling countries and has in recent weeks sanctioned a drip feed of extra emergency finance to Greece's lenders.
Greek Finance Minister Yanis Varoufakis last week criticized ECB policy towards Athens as "asphyxiating," a criticism also made by the protest organizers. (Additional reporting by Paul Carrel; Editing by Erik Kirschbaum and Louise Ireland)