Several police cars have been set on fire, with windows being smashed and demonstrators throwing stones at police ahead of the massive demonstration on Wednesday.

Authorities say at least one officer has been injured by a stone hurled by an activist, near the city's opera house.

Up to 350 people were detained by police.

Organizers have accused the police of sparking the violence, saying they set up a "civil war type scenario" to provoke demonstrators.

"This is not what Blockupy planned," spokesman Hendrik Wester said.

Video footage has shown riot police running through the city, with at least one protester being dragged away.

The chairman of the German police union (GdP), Oliver Malchow, says the violence has gone far enough.

"We're talking about serious crimes here," he told broadcaster n-tv.

"The term 'protest' doesn't fit in this case," he said.

Activists are targeting the European Central Bank (ECB) as a symbol of capitalism, as well as over its part in restrictive austerity measures in financially-troubled countries such as Greece.

Around 100 guests, including ECB President Mario Draghi, are expected to attend the opening of the ECB's new headquarters.

Authorities had erected a security zone around the 185-meter skyscraper, putting up barricades and barbed wire in preparation for the protest action.

A police spokeswoman said, "It is one of the biggest deployments ever in the city." She said the authorities expected the majority of demonstrators to be peaceful. But they believed some violence-prone protesters could use the crowds as cover, and had placed helicopters and water cannons on standby.

Around 10,000 anti-capitalist protesters, marching under the banner of leftist alliance Blockupy, are expected to attend the rally, with a march through the city planned for later in the evening.

A special train has been chartered by organizers to bring 800 people from Berlin, and 60 buses from 39 European countries are also heading to the financial hub.

Demonstrators have said they want to blockade the billion-dollar building, in an attempt to disrupt what they describe as "capitalist business as usual."

The ECB, along with the European Commission and the International Monetary Fund, are part of the so-called "troika" monitoring countries' compliance with the conditions of bailout loans.

These include lowering government spending and other debt-reduction methods, which have been blamed for increasing unemployment and stunting economic growth.

The ECB says it plans to remain "fully operational" despite the action, with some employees working from home.

an/bw (AP, AFP, Reuters, dpa)