Twelve police officers and several protesters were injured overnight Friday when anti-Nazi activists tried to rush a far-right rally in the northern port city of Hamburg, police said.

More than a dozen people were arrested, a police spokesperson added.

Violence erupted when 500 to 600 demonstrators tried to break past a police line to attack the 100-odd members of the NPD party who were gathering for an open-air rally.

One police officer fired a warning shot with his service pistol when the demonstrators shattered the windows of the vehicle in which he was in, the spokesperson added.

Nearly 2000 police officers were deployed overnight, and more unrest were feared on Saturday during an annual festival in an alternative district of Hamburg.