Thirty-nine people were detained after police were attacked with “a hail of bottles” at an open-air party in central Frankfurt attended by thousands of youngsters, AFP quoted police in the German city as saying on Sunday.

Five officers were injured in the riot that began at around 3am when police intervened to stop a brawl involving around 30 people in Frankfurt's historic Opera square.

The square has become a popular gathering place for what local media have dubbed “corona parties” as Germany's bars and clubs remain closed to contain the spread of the virus.

Some 3,000 mainly young people filled the square again on Saturday evening but only around 500 to 800 were still there when the unrest started, Frankfurt police chief Gerhard Bereswill told a press conference.

A small group of officers moved in to help a bleeding man and end the brawl, but the crowd turned on them. “What I find especially abhorrent is that bystanders cheered and clapped when bottles hit my colleagues,” Bereswill said.

A total of 39 arrests were made, including one woman.