The police were alerted at 22.21. Several people called and told us about a loud bang in the area, which is a few hundred meters east of the railway station.
Police National Bomb Protection (NBS) and forensic technicians have been called to the scene.
Police say on their site that damage has been found on a building adjacent to the square. According to Kvällsposten, this is a gate.
No injured person was found.
The police have blocked off Östergatan, Drottningtorget and access streets to the square. The square has also been vacated. Several bars and restaurants are located on the square.
The incident is classified as a general destruction.