At least 16 people are feared dead after a bomb ripped open buildings in the heart of Norway's government and a man dressed as a police officer opened fire at an island youth camp connected to the ruling party.
Seven people were killed in the blast which hit government buildings in the capital Oslo, while at least nine died in the camp shootings on the island of Utoya.
David Cameron said the attacks are a "stark reminder of the threat we all face from terrorism", and he immediately vowed Britain's assistance to Norway in tracking down those responsible.
Eyewitness Andre Scheie told Norwegian broadcaster NRK that he saw "very many dead by the shore", possibly up to 25, at the camp on Utoya, where the youth wing of the Labour Party was holding a summer camp for hundreds of youngsters. He said some victims were shot in the water after trying desperately to swim away.
One man - a Norwegian - has been arrested following the camp shooting, and officials suspect he is linked to the Oslo blast as he was spotted in the city before the explosion.
In the Norwegian capital, a square was covered in twisted metal, shattered glass and documents expelled from surrounding buildings, which house government offices and the headquarters of some of Norway's leading newspapers.
Most of the windows in the 20-floor high-rise where Prime Minister Jens Stoltenberg and his administration work were shattered. Mr Stoltenberg was working at home at the time and was unharmed.
Police said the explosion was caused by "one or more" bombs, but declined to speculate on who was behind the attack. They later sealed off the nearby offices of broadcaster TV 2 after discovering a suspicious package.
Public broadcaster NRK showed video of a blackened car lying on its side amid the debris.
Mr Stoltenberg told Norwegian broadcaster NRK: "Co-workers have lost their lives... it's frightening. That's not how we want things in our country. But it's important that we don't let ourselves be scared. Because the purpose of that kind of violence is to create fear."