Norway has been hit by twin attacks - a massive bomb blast in the capital and a shooting attack on young people at a governing Labour Party youth camp.
At least seven people were killed in the bombing, which inflicted huge damage on government buildings in Oslo.
A few hours later a gunman opened fire at the camp on an island outside Oslo, killing at least 10.
The suspected gunman was arrested at the camp and the government have confirmed that he is Norwegian.
Police have said the 32-year-old suspect was also linked with the bomb attack.
Witnesses described the gunman as tall, blonde and say he was dressed as a policeman.
'Shaken by evil'
Prime Minister Jens Stoltenberg, whose Oslo offices were among those damaged by the bomb, described the attacks as "bloody and cowardly".
He said Norway had been "shaken by evil" but that Norwegian democracy and ideals would not be destroyed.
"We are a small nation and a proud nation. No-one will bomb us to silence, no-one will shoot us to silence," he said in a televised address.
No group has claimed responsibility for the attacks.
There are fears the number of dead from both attacks could rise, says the BBC's Richard Galpin, north of Oslo.
Several people from the camp are still missing and rescue teams have been scouring the waters around the island after dead bodies were reportedly seen in the water.
Eyewitnesses say that after the gunman started shooting, people jumped into the water to try to escape the hail of bullets.
There are concerns more victims may still be inside buildings hit by the initial explosion.
Emergency services have had difficulty accessing these buildings amid concerns about further possible explosions as well as fears the blast may have left buildings unstable.
'Posed as policeman'
The gunman is reported to have been armed with a handgun, an automatic weapon and a shotgun.
"He travelled on the ferry boat from the mainland over to that little inland island posing as a police officer, saying he was there to do research in connection with the bomb blasts," NRK journalist Ole Torp told the BBC.
"He asked people to gather round and then he started shooting, so these young people fled into the bushes and woods and some even swam off the island to get to safety."
One 15-year-old eyewitness described how she saw what she thought was a police officer open fire.
"He first shot people on the island. Afterward he started shooting people in the water," youth camp delegate Elise told the Associated Press news agency.
Mr Stoltenberg had been due to visit the camp on Saturday. Foreign Minister Jonas Gahr Store, who visited the camp on Thursday, praised those who were attending.
"The country has no finer youth than young people who go for a summer camp doing politics, doing discussions, doing training, doing football, and then they experience this absolutely horrendous act of violence."
In Oslo, government officials urged people to stay at home and avoid central areas of the city.
Rubble and glass from shattered windows littered the streets and smoke from the fires drifting across the city could be seen in television footage from the devastated government quarter.
Earlier Egil Vrekke, Assistant Chief Constable of Oslo police, told the BBC the rescue operation in Oslo was ongoing, with large areas still cordoned off as bomb experts established whether there were other devices in the area.
Friday was a public holiday in Norway so although there were hundreds of people in the government offices hit by the blast, they were not as busy as they might usually have been, said State Secretary Kristian Amundsen.
"We have to focus on the rescue operation - there are still people in the building, there are still people in the hospital," he told the BBC.
The oil ministry was reportedly among the government buildings hit, while the headquarters of tabloid newspaper VG were also said to have been damaged in the blast, which was heard across the capital.
"It's complete chaos here. The windows are blown out in all the buildings close by," NRK journalist Ingunn Andersen told AP.
The US has condemned the "despicable acts of violence" in Oslo, while the President of the European Council, Herman Van Rompuy, said the "acts of cowardice" had no justification.
Our correspondent says that the attacks have been a huge shock for people: Norway has never experienced anything like this in the past and the violence of the past day has left people totally stunned.