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.
There are as yet no further details about what triggered the rampage. Reports say hundreds of people were involved.
The police statement said some rioters threw chunks of pavement 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. Police say several shops were looted.
The situation was calmer after dawn and firefighters began repairing the many damaged shop fronts.
The police say they are questioning 20 people they detained in the trouble, and they have appealed for witness evidence including mobile phone footage. However, a number of the rioters reportedly wore helmets or masks to conceal their faces.