Experts and witnesses described on Tuesday in often horrifying detail the massive blast that rocked Oslo when Anders Behring Breivik bombed a government building last July, killing eight people.

"The body was totally crushed," testified Arne Stray-Pedersen, Norway's chief medical examiner, describing the results of one of four autopsies, using anatomy sketches and photos of blast projectiles.

Government employees and passers-by were hit by the massive blast from an explosives-laden van parked outside the building that houses the offices of Labour Prime Minister Jens Stoltenberg, who was not there at the time.

Eight people died and nine others were seriously injured in the blast.

"In the government district we found several hundred body pieces," criminal technician Ole Morten Stoerseth told the Oslo district court.

In the courtroom, families of Breivik's victims stifled sobs and embraced, but the accused himself showed no sign of emotion upon hearing details of his bloodbath, as has been the case since his trial started more than a week ago.

On July 22nd, Breivik had placed a 950-kilo bomb, made from fertilizer, diesel and aluminium, in a van that he parked at the foot of the 17-floor government tower.

The 33-year-old right-wing extremist has said the bombing, and the later shooting of 69 people on Utøya island, were "cruel but necessary" to stop the Labour Party's "multicultural experiment" and the "Muslim invasion" of Norway and Europe.

Tor Inge Kristoffersen, a guard in the Norwegian capital's government block, told the court Tuesday how he had seen a white van park in front of the entrance and had begun using surveillance camera images to check whether it was authorized to be there.

"When I was zooming in on the number plate, the car exploded," he testified, adding that "half of the images disappeared from our screens because the cameras had been destroyed in the explosion."

"There was a huge roar. We were so close that we did not hear a blast, but a roar, and we noticed the shockwave in the ceiling over us," he said.

Kristoffersen, who served with the Norwegian military in the Middle East and in the Balkans, continued to work in the government district after the attacks, and said the area had been like "a war zone".

In the weeks after the twin attacks many raised questions about how the right-wing extremist could have parked his van so close to Norway's political nerve centre.

Kristoffersen stressed that long-overdue construction was under way to block off traffic in the street outside the government building, but that in the meantime "illegal parking" was frequent in the area.

"We chased cars away from there every day," he said.

Svein Olav Christensen, a government explosives expert, meanwhile told the court that a reenactment and simulations showed that Breivik's bomb had the energy equivalent of between 400 and 700 kilos of TNT.

"The main charge is easy to make," he said, adding though that "the detonator is more difficult."

Police operation chief Thor Langli was also called to testify Tuesday and described the confusion that followed the blast, with contradictory messages suggesting there were two suspects and possibly other bombs ready to go off.

"I thought there was a connection," he said about the moment when he was told about the Utøya massacre.

"I could not conceive that we could be facing several guys like him at the same time," Langli said, turning towards the accused.

Breivik has been charged with "acts of terror" and faces either 21 years in prison -- a sentence that could be extended indefinitely if he is still considered a threat to society -- or closed psychiatric care, possibly for life.

Breivik himself wants to be found sane and accountable for his actions, so that his anti-Islam ideology, presented in the 1,500-page manifesto he published online just before the attacks, will be taken seriously and not considered the ravings of a lunatic.