Rioters looted several shops in Frankfurt late on Saturday and clashes erupted there and in other German cities after largely peaceful mass protests over the cost of living.
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.
Authorities in Frankfurt, Munich and Berlin reported as many as 18,000 protesters, roughly half of what the organisers had estimated on their website.
Gen Z regelt spokesperson Lina Friedrich said that the demonstrations were "to fight for peace and freedom, rather than for war and coercion".
She said: "What matters most to us is better support for the health and education systems. We are against compulsory military service and we are in favour of a fairer distribution of the burden."
While protests during the day were largely peaceful, the demonstrations turned violent as night fell.
In a post on X, Frankfurt police said that "violent riots" were occurring in the city centre. The windows of several shops had been smashed and molotov cocktails were thrown at emergency services, police said.
In later updates, officials said the windows of a gun shop were smashed and that they had seized two weapons after arresting a suspect.
Roman Poseck, interior minister of Hesse state, of which Frankfurt is the capital, said that "around 100 clearly identified left-wing extremists" had infiltrated the protest in the city. He added that it was "shameful" how the protests had ended.
Poseck said: "We all have those images from France in our minds – looting, burning houses, burning police vehicles, and attacks on police officers... France must not be held up as a model in this regard."
