Police arrested 39 people in Frankfurt in the early hours of Sunday morning after an open air party in the city centre ended in violence.
Large numbers of police in riot gear were seen advancing near the city's Opernplatz, which has been routinely used for public partying by thousands of people in recent weeks.
Police reportedly became involved after a reveller appeared to be injured after the mood soured and a mass brawl broke out in the early hours.
Officers attempting to help were reportedly turned on by multiple party-goers who hurled bottles and glasses at them while bystanders applauded and chanted anti-police slogans.
Authorities subsequently cleared the square, with the incident set to be discussed at a security conference in the city on Monday.
Last month rioting and looting also took place in Stuttgart when several hundred people rampaged through the city after police performed checks on people suspected of using drugs.