Frankfurt police said they used water cannons to disperse protesters after attacks on officers, firefighters, a police station and the Old Opera House.
At least 94 officers have been wounded, police said. Some suffered injuries when stones were thrown at them, and about 80 were hurt when they were attacked with a substance similar to pepper spray, police said.
Those affected by the substance had respiratory problems and eye irritation and needed treatment, police said. The substance quickly dissipated, and the place where it was discharged was not cordoned off.
The damage included at least seven police cars that were set on fire.
Officers arrested five people for violent acts against police and detained another 500 people for questioning, Frankfurt police spokeswoman Tessa Koschig said.
The situation had calmed down by early afternoon Frankfurt time, but police said they feared further violence in the evening, when the main protest against the ECB is scheduled to take place.
The protest movement, dubbed "Blockupy," comes amid criticism of austerity measures from the Greek finance minister, who said his country is suffering under the ECB's policies.
"We want the European Central Bank to stop the austerity in Europe that is responsible for mass joblessness. ... People don't have enough to eat," Blockupy spokeswoman Songa Winter said.
The European Central Bank is supposed to ensure price stability in the eurozone
, and it tries to keep inflation levels just below 2%. The region has been suffering from depressed economic activity, and unemployment remains near record highs
"What we're seeing, I think, in Frankfurt is a reaction you see across the eurozone
for many people who have said it's just been too painful, too much, and it's been going for too long," CNN's Jim Boulden said.
A Blockupy spokeswoman said the group does not condone the clashes that erupted.
"Blockupy is clearly against violence," Frauke Distelrath said. "Obviously, there are people that go against what we intended for this day. It is not what we planned, but it shows people are very angry about the austerity policies."
Blockupy is a coalition of more than 90 groups from across Europe, including the Syriza Party, which now governs Greece.