The bomb went off when the UN's Baghdad headquarters was at its busiest, according to one of the UN staff in the building at the time.
Many people were feared trapped in the rubble
There were, Fouad Victor said, many people inside because "no one had gone home yet".
Alice Yacoub, another UN employee, was sitting in the cafeteria when the blast happened.
"Everything came down on our head," she said. "I can't find my
colleagues and I'm worried about them."
Fayez Sarhan, another employee, saw what he said was the actual bomb vehicle - a yellow cement lorry - smashing into the former Canal Hotel's wall.
The UN's spokesman at the scene, Salim Lone, was at his computer when the explosion happened:
"There was a huge, shattering explosion. I was working on the computer, the glass in the window blew - the woodwork, everything came out of the roof.
"I ran into the corridor. Everybody was very severely wounded, very badly hurt, bleeding, people with blood on their face."
Mr Lone described the extent of the damage, particularly in the area around the office of his boss, UN special envoy Sergio Vieira de Mello.
"I'm looking at it right now and there is his office and the offices underneath him and around him no longer exist. It is all rubble.
"Certainly half of this huge building has been badly damaged, because I am about 60 yards [metres], 70 yards, away from the office that the explosion occurred under and I thought that the bomb was right behind me.
"And you came out and, oh, the damage is everywhere."
As bloodstained survivors walked away from the rubble, they talked of their fears amid the initial confusion.
"I saw many people killed and injured inside the building," said Adnan al-Jabouri, a UN driver.