A powerful bomb has torn open several government buildings in Oslo, including the prime minister's office, killing two people.
At least 15 people have been injured and government official Hans Kristian Amundsen told the BBC some people are trapped at the bomb site.
Norwegian PM Jens Stoltenberg is safe, as he was working at home at the time of the blast.
"There has been one or several powerful explosions in the government district in Oslo," city police said in a brief statement. "So far, police cannot say anything about the scope of the damage, aside from that there's been one or several explosions."
Broadcaster NRK showed video of a blackened car lying on its side amid the debris.
Most of the windows in the 20-floor building have been blown out, and the bottom floor appears to be gutted. Nearby offices are also heavily damaged and have been evacuated, including those housing some of Norway's leading newspapers and the news agency NTB.
Meanwhile, five people are believed to have been shot after a gunman opened fire at Norway Labour Party's youth camp outside Oslo
Labour spokesman Per Gunnar Dahl said a man dressed in a police uniform started shooting at youths assembled for the party's annual youth camp at Utoya, an island outside Oslo.
He said unconfirmed reports were that five people were hit. "We don't know how serious," he added. He said around 700 people, mostly teenagers between aged 14-18, were assembled for the camp.
One person has been arrested.
