Two people were killed and 14 others injured in a gunshot near a gay bar on the eve of the Pride parade in Oslo.
According to the police, a suspect was arrested in the Norwegian capital on Saturday night.
Everything points to a single perpetrator, said police chief Tore Barstad.
However, the police forces in Oslo have been strengthened as a precaution.
Barstad initially did not want to comment on a possible terrorist background to the act.
The shots were fired in the center of the Norwegian capital near the gay club "London Pub" and a jazz club, as well as a snack bar.
Police representatives discussed during the night whether the planned Pride march on Saturday afternoon can take place as planned.
"Sure what he's aiming for"
"He seemed very sure of what he was aiming at," said an eyewitness to the newspaper "Verdens Gang" about the perpetrator.
Another witness told the newspaper that the man had fired an automatic weapon and that the crime scene looked "like a theater of war".
Several people with head injuries were lying on the ground.
Heavily armed police officers in riot gear secured the crime scene.