Groups of people have smashed shop windows, looted and attacked police vehicles in central Stuttgart during hours of night-time disturbances.
German police say more than a dozen police officers were hurt during the violence, in south-west Germany.
Video clips on Twitter show people vandalising shops in the city centre and hurling big stones and other objects at police vehicles.
Stuttgart police say the violence began after police checked a drugs incident.
Their inquiries into alleged drug-dealing among youths at a late-night street party appears to have sparked an aggressive reaction.
Local politicians and ordinary citizens expressed shock, saying the scale of the rampage was unprecedented for Stuttgart in recent years.
A mobile phone shop and a jewellery store were among the businesses attacked and looted. Some fast food outlets were also vandalised, public broadcaster SWR reports.
There are as yet no details about those involved. Reports say as many as 500 rampaged, many of them hooded and masked.
The police statement said some rioters threw chunks of paving stone and other missiles at passing police cars. At one point, the police said, the situation was “out of control”.
More than 200 extra police were rushed into the city centre, where shops had been attacked apparently at random.
The situation was calmer after dawn and firefighters and civil protection volunteers (THW) began repairing the many damaged shop fronts.
The police say they are questioning 20 people they detained and they have appealed for witness evidence including mobile phone footage.
The city’s mayor Fritz Kuhn, of the Green Party, tweeted: “I’m shocked by the outbreak of violence, the attacks on police and destruction in our city. It’s a sad Sunday for Stuttgart.
“We’re analysing what happened carefully. One thing is clear: Stuttgart can have no lawless spaces.”
The interior minister of Baden-Württemberg region, Thomas Strobl, said “the riots we had to witness last night in Stuttgart were of a kind we have never experienced previously in Baden-Württemberg”.