The act was carried out near several bars, mainly London pub and Per på Hjørnet, at C J Hambro’s pass in central Oslo at 01:15, the night before Saturday.
The perpetrator was stopped by people on the spot. He was arrested by police at 1:19 p.m.
21 people were injured, of which about ten were serious but not life-threatening.
The man is a Norwegian citizen but has an Iranian background. He had two weapons, one of which was an automatic weapon of an older model.
The man has previously been known to the Norwegian security police PST (Police Security Service) since at least 2015, and he is suspected of having been part of an extremist Islamist network in Norway.
PST conducted a conversation with the man in May, when he said that he had no thoughts of carrying out violence.
The man’s mental health has previously been highlighted in several lawsuits.
The man’s lawyer, John Christian Elden, expects the man to undergo a forensic psychiatric examination. This was endorsed by police prosecutor Børge Enoksen.
PST assessed the mass shooting as an extremist Islamist terrorist act and raises the threat level in Norway to the highest.
The organizers of the pride parade in Oslo, which was to be held on Saturday, decided to cancel the parade after consultation with the police and PST.
Norwegian Prime Minister Jonas Gahr Støre, the Labor Party, said at a press conference on Saturday afternoon that they do not believe there are further perpetrators.