This is a file photo of police officers patrolling in Orebro, Sweden.

Swedish police say riots broke out overnight in a predominantly immigrant Stockholm suburb after officers arrested a suspect on drug charges.

Spokesman Lars Bystrom said Tuesday that unidentified people, including some wearing masks, threw rocks at police, set cars on fire and looted shops in Rinkeby. One officer was slightly injured when hit by a rock. No arrests were made.

Bystrom says one officer fired his gun, not as a warning shot but because he was "in a situation that demanded he used his firearm."

Bystrom added that "no one was hit," but declined to give further details, saying the episode would be investigated.

Bystrom said: "This kind of situation doesn't happen that often but it is always regrettable when they happen."