The ceremony went ahead as planned, with ECB President Mario Draghi saying the new building for the euro bloc's monetary authority was "a symbol of what Europe can achieve together."
Police said 350 people were detained as officers pursued what they said was a minority of violence-minded activists using the protest as cover. The bulk of demonstrators conducted themselves peacefully, marching in groups, drumming and singing ahead of a rally in the city's main square. Some blocked bridges across the Main River or streets in an effort to hinder access to the ECB ceremony.
Protesters said they are targeting the central bank because of its role in supervising efforts to restrain spending and reduce debt in financially troubled countries such as Greece. Blockupy activist Hannah Eberle said that "people remain determined" despite the heavy police presence. "The mood is good, we are in solidarity with each other," she said ahead of the rally: "The day is long, we are not yet finished."
At the ECB ceremony, Tarek Al-Wazir, economy minister of the German Hesse region, called on the peaceful demonstrators to "distance yourselves from the violence."
Wazir, a member of the Greens party, said that the protesters "have no answers... but they have some of the right questions," conceding that "austerity can indeed be self-defeating."
He said the ECB was the wrong target for this kind of protest. Draghi has in fact urged more spending by governments that are in good financial shape such as Germany to help lift the economy and reduce unemployment. That advice has been ignored by the German government, which has stressed the need to balance its budget and pressed others to restrain spending.
Police said several police cars were set on fire at a police station in the city centre. Another police vehicle smouldered a block from the ECB. Private vehicles and garbage bins were also torched.
Some 10,000 people were expected on Frankfurt's main square, the Roemerberg. Participants were to include trade unions and Germany's Left Party.
The ECB, along with the European Commission and International Monetary Fund, is part of the so-called "troika" that monitors compliance with the conditions of bailout loans for Greece and other financially troubled countries in Europe. Those conditions include spending cuts and reducing deficits, moves that are aimed at reducing debt but have also been blamed for high unemployment and slow growth.
Anti-austerity activists received a political boost when Greece's left-wing Syriza party won elections there in January by campaigning against the bailout deal and its conditions, which they say has led to a "humanitarian crisis."