Those who heard the explosion described it as sounding like thunder breaking immediately overhead; some ducked instinctively. Then, unimaginably, it began to rain glass. Oslo, one of the most civilised of capital cities, was under attack.
The massive blast shook the centre at 3.26pm local time on Friday (2.26pm BST), laying waste to the administrative heart of the government, blowing out the windows of the Prime Minister's offices, devastating the finance and oil ministries and killing at least seven people.
The boom was heard across the city. The area targeted, the Regjeringskvartalet, the epicentre of government, is near the Stortinget (parliament building) and a brisk walk from City Hall, where the Nobel peace prize is awarded annually.
Witnesses described the immediate aftermath as like a war zone, with survivors walking dazed through streets strewn with rubble. A huge dust cloud billowed up over the city centre, spreading a shroud of dust and debris beyond its boundaries.
The force of the explosions blew out nearly every window in the 17-storey office which contains the office of the Prime Minister, Jens Stoltenberg, as well as the justice ministry. Across the road, fires started inside the oil and finance ministries.
Former journalist Andreas Lunde, who was among the first on the scene, said: "When I walked around the corner, it looked like a war zone, a catastrophe. There was glass, smoke, fire, an area of catastrophe.
"I saw a man give someone heart compressions on the ground, but the person on the ground was definitely dead, so I stopped him and found something to cover the face.
"I saw several injured people, but if it looked like they were being cared for, I walked on. When I saw a person who was missing both legs, before someone came, I covered that person up too."
Ben Symes, a British student who was in Oslo, said: "It came out of the blue. We felt this huge explosion then a powerful force through our legs.
"I don't know what I first thought. I looked out of the shop window and saw shattered buildings and carnage everywhere. Ours was the only one which seemed to be still intact.
"There was smoke and ash everywhere, loads of debris and dust in our eyes. We walked and saw huge slabs of concrete and bits of road all torn up. I saw a car which was completely blown out and overturned. Then I knew what happened."
Police rapidly cordoned off the area and evacuated nearby buildings. Ambulances rushed dozens of injured to hospital. Experts said that the toll of victims would almost certainly have been higher but for the fact that many people had headed home for the weekend. Among them was the Prime Minister, who was working from home when the explosion struck.
Amid the wreckage and devastation were the steel skeletons of two cars parked close to the government offices which bore the brunt of the explosion. Norwegian police said yesterday that they believed one had been parked at the scene at least one hour earlier.
The man they suspect of parking it is Anders Behring Breivik, a 32-year-old Norwegian businessman, who lived in an apartment with his mother in the wealthy west Oslo district of Skoyen, a 10-minute drive away from the scene.
They believe the vehicle had been packed with a lethal mixture of ammonium-nitrate fertiliser bags and fuel oil. Similar bombs have been used to devastating effect by terrorist groups all over the world. In 1992, a so-called Amfo bomb planted by the IRA exploded in Bishopgate, in the City of London, killing three.
Yesterday, Oddny Estenstad, a spokeswoman for an agricultural supply company, confirmed that Breivik, who runs a farm company, purchased six tons of fertiliser 10 weeks ago.
As police and rescue workers picked their way through the rubble searching for trapped victims, Breivik was believed to be travelling on the E18 road towards Tyrifjorden lake, some 38km north-west of Oslo. His destination was Utoya island where a summer youth camp organised by the ruling Labour Party was in progress.
On board the ferry, Breivik, who was wearing a blue uniform, passed himself off as a police officer, saying he was a security follow-up in the aftermath of the Oslo bombing. When he disembarked at 5.10pm, witnesses described how he called the teenagers to him. One, Hana, a 16-year-old, later said: "I saw a policeman stand there with earplugs. He said: 'I'd like to gather everyone.' Then he ran in and started shooting."
Eyewitnesses described chaotic scenes that ensued as terrified teenagers fled from the gunman, some plunging into the water to swim to safety. Others hid in the undergrowth, cowering in fear.
What followed was more than an hour of carnage as Breivik stalked teenagers across the island, shooting at them with at least three weapons, including a Glock handgun, a shotgun and a semi-automatic rifle, pausing only to reload each time his ammunition ran out. Those who plunged into the lake trying to escape were shot as they tried to swim to safety; others played dead, while some tried to hide behind rocks on the shore.
"I just saw people jumping into the water, about 50 people swimming towards the shore. People were crying, shaking, they were terrified," said Anita Lien, 42, who lives by Tyrifjorden lake. "They were so young, between 15 and 19 years old," she said.
Erik Kursetgjerde, an 18-year-old Labour Party youth member, described the fear and panic as the gunman methodically carried out the massacre. The killer, dressed as a policeman, "would tell people to come over: 'It's OK, you're safe. We're coming to help you.' He said. "I heard screams. I heard people begging for their lives and I heard shots. He just blew them away. I was certain I was going to die.
"And then I saw about 20 people come towards him and he shot them at close range," he said. Mr Kursetgjerde said he hid between cliffs, then swam out into the lake and nearly drowned. "Someone [in a boat] rescued me. They saved my life."
Thorbjoern Vereive, 22, said: "Some people said he also had something that looked like a rifle. When I swam out into the water, he shot all my friends. He shot them when they were running away. I hid in a cave and had to lie in the water.
"He picked them out, one by one. He was mostly silent. It looked like it didn't bother him. He tried to shoot me and I saw the bullet pass right next to me. I thought I was going to die."
Jorgen Benone said: "I was hiding behind some stones. I saw him once, just 20-30 metres away. I thought of all the people I love. I saw some boats rescuing people but I wasn't sure if I could trust them. I didn't know who I could trust any more."
Police, alerted by scores of desperate mobile phone calls from teenagers hiding from Breivik, struggled to respond rapidly. With resources focused in central Oslo, they struggled to get a trained team of armed officers to the island. Yesterday a police spokesman admitted that the massacre continued for at least an hour and 35 minutes before the armed response team reached the island.
When the Swat team arrived by helicopter, the gunman surrendered, said police chief Sveinung Sponheim. "There were problems with transport to Utoya. It was difficult to get boats, but that problem was solved when the Swat team arrived."
Only after Breivik was captured could they begin to count the human cost of what experts are describing as the deadliest attack by a gunman. By that time, at least 85 people were dead. Last night police admitted that four or five people were still missing. Divers have been searching the waters around the island.
Police said Breivik is talking to them and has admitted to firing weapons on the island. It was not clear if he had confessed to anything else.
Police said he had retained a lawyer, but the attorney did not want to be named. "He has had a dialogue with the police the whole time, but he's a very demanding suspect," Mr Sponheim said. Police were unable to confirm reports that a second gunman was present on the island. Mr Sponheim said detectives are still investigating those claims.