Rioters looted shops in central Frankfurt late on Saturday as clashes erupted in German cities 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 targeting emergency workers with fireworks, stones and bottles, reports the BBC.
Thousands joined largely peaceful demonstrations earlier in the day. They were 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.
As night fell, demonstrations turned violent.
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.
Authorities in Frankfurt, Munich and Berlin reported as many as 18,000 protesters, roughly half of what the organisers had estimated on their website. German media say tens of thousands protested in German cities on Saturday.
Gen Z regelt spokesperson Lina Friedrich said 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."
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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 biggest city, 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."
The protests in Germany took place on the same day as other large-scale demonstrations in Europe.
In France, more than 320,000 people took part in a third consecutive weekend of largely peaceful protests in cities across the country, demanding action on the high cost of living and poor public services, especially in the education sector.
Thousands of police officers were deployed across France to deal with the demonstrations, and officers fired tear gas at demonstrators in some cities.
There were more than 500 arrests after clashes in Toulouse, Lyon and Marseille.
French Interior Minister Laurent Nuñez said a police officer had been injured by a molotov cocktail. Police in Paris said several hundred rioters had attacked security forces with projectiles.
Protests also took place in Italy, with some clashes between police and demonstrators in Rome and Turin.
In Portugal, police moved to clear a new tent camp assembled after thousands marched to protest against a rental bill that they argue could worsen a housing crisis in the country, where house prices have more than doubled over the past decade.
Mostly young protesters have recently taken to the streets in a swathe of European nations: France, Belgium, Ireland, Spain, Portugal, Italy, Greece, Serbia and Albania.
While the specific demands vary from country to country, the protesters have largely focused on cost-of-living issues, such as access to housing, the quality of education and cuts to public services.