His defence lawyer Geir ­Lippestad last night said of Breivik: “He has said that he believed the actions were atrocious, but that in his head they were necessary.”

On Saturday night a video emerged in which the killer, posing with weapons, appears to set out his motivation for the attacks, calling for the eradication of Islam and Marxism from Europe.

Picture of killer from scene

Breivik shot several teenagers as they tried to swim off the island to safety. Police teams were searching the water and rocky inlets looking for more corpses.

A mini-submarine was called in to help the search for bodies, along with divers.

Survivors told how they hid under bunk beds, behind rocks and in cabins as Breivik, dressed as a police officer, beckoned the youths to him, promising them safety. Youngsters who fell for his ploy were shot in cold blood. Survivors told how they had heard people plead for mercy. Thorbjørn Vereide, 22, who ran away and hid in a cave, said: “He seemed very focused. He took his time and picked victims out one by one. People lay on the ground, and he went over them and shot them in the back. He shot them all twice to make sure they were dead.

“He kept shouting: ‘It’s safe to come out. You’ll be saved. I’m a cop.’”

Nicoline Bjerge Schie, 21, who cowered behind a rock near the beach, said: “I could not see the gunman but I heard him screaming and laughing and he gave several cheers.”

She watched at least five of her friends being hit by the gunman’s bullets and watched as the bodies tumbled off the rock and into the lake.

Adrian Pracon, 21, who was shot in the shoulder, said from his hospital bed: “He was yelling out that he was going to kill us all and that we all must die.” Mr Pracon played dead but survived.“He tried everyone, he kicked them to see if they were alive, or he just shot them,” he said.

Erik Kursetgjerde, an 18-year-old Labour Party youth member, said Breivik “would tell people to come over: ‘It’s OK, you’re safe, we’re coming to help you.’ And then I saw about 20 people come towards him and he shot them at close range.”

Edvard Foernes, 16, said the gunman walked through the camp, saying: “Come out and play with me. Don’t be shy.”

Rescuers told of the agonising decisions they had to make as they headed to the island in boats to collect children trying to swim to safety.

Torill Hansen, who was camping nearby, said: “I could only take 10 people in the boat and even with that many it was nearly capsizing. Having to decide who to take was horrible.”

Breivik surrendered to an armed, specialist police unit which said it had arrived on the scene about 40 minutes after being called out by youngsters at the camp, organised by the youth wing of Norway’s ruling Labour Party. By then, Breivik, who had two guns when he was arrested, had been shooting for 90 minutes without anybody able to stop him.

Breivik, who surrendered without firing a shot, had undergone military training as part of his compulsory national service and held licences for two weapons including a Glock semi-automatic pistol.

He was also a member of Oslo’s Masonic lodge and posted pictures of himself on the internet in the masonic regalia.

A keen body builder and gun enthusiast, he had held several positions in one of Norway’s biggest political parties, the Right-wing Progress Party, from 1999 to 2007.

His views had become increasingly extreme in recent years and he had been seen by neighbours wearing paramilitary uniform.

Writing on the internet, he cited his hatred for Muslims and enthusiasm for the English Defence League. On the social networking site Twitter Breivik posted a quote on July 17 by the English philosopher John Stuart Mill: “One person with a belief is equal to the force of 100,000 who have only interests.”

It emerged on Saturday he had run a farming business and only 10 weeks ago had bought six tons of artificial fertiliser, which he is believed to have used to make the car bomb that was detonated in Oslo’s political district.

The suppliers thought nothing of selling such an amount to a farm businessman. After the bomb exploded, it appears Breivik drove a silver-grey van to Utoya. The van, recovered by police on Saturday, also contained explosives.

Acting Police Chief Sveinung Sponheim said: “It’s very difficult at this point to say if he was acting alone or if he was part of a larger network.”

Police have refused to name the victims but said two members of the government were killed in the blast. Norway’s royal family and prime minister led the nation in mourning on Saturday, visiting grieving relatives of the dead youths.

Oslo Cathedral became home to a makeshift shrine with hundreds going there to lay flowers and light candles.

Fighting back the tears, Jens Stoltenberg, the prime minister, said: “It was a paradise of my youth that has now been turned into hell.”

King Harald said: “I’m horrified at the rising toll of fatalities. In the midst of this all we have seen a prime minister handling the situation in a remarkable way.”