Around 50 Hells Angels armed with sticks stormed a red-light district bar called The Fat Mexican, which is frequented by rival biker gang the Bandidos, police reported.
The bar was smashed to pieces during the ensuing brawl and several people were injured before police managed to herd everyone outside and separate the two gangs.
News agency AP reported that several hours later someone lobbed a hand grenade through the window of the Hells Angels' clubhouse in Solingen, where about 20 people were inside.
The grenade did not go off, and police later detonated the device in a controlled explosion.
A police spokesman said he could "not rule out" the possibility that the Bandidos were behind the attack.
Fears of an outbreak of violence between the two biker gangs were sparked in October when a 32-year-old man was shot dead outside the Fat Mexican – which is on the ground floor of the Bandidos' club building. The dead man was a Bandido, while the suspected killer is a man with connections to the Hells Angels.