Rioters looted shops in central Frankfurt late on Saturday and clashes erupted with police there and in other German cities after largely peaceful mass protests over the cost of living.
Besides Frankfurt - Germany's financial hub - there were incidents of violence in Berlin, Cologne and Hamburg, with police saying they used water cannon to disperse protesters who targeted emergency workers with fireworks, stones and bottles.
Thousands earlier joined the demonstrations organised by the campaign group Gen Z regelt - meaning "Gen Z takes charge" - to protest against underfunding in schools, the high cost of living, public spending cuts and military service.
The unrest in Germany follows similar youth-led mass protests in France, where more than 320,000 people also demonstrated on Saturday.
A large section of Frankfurt's main shopping street was closed off by police, AFP news agency reported. A police spokesperson said many arrests were made in the city.