The devastating bomb attack on the United Nations headquarters in eastern Baghdad brought one side of the building crashing down on the employees working in offices inside.
Minutes after the explosion UN staff were rushing out in a state of shock as a huge plume of black smoke rose from cars that had been hit in the explosion.
"I have seen so many friends inside, they are all injured," cried one woman as she rushed past almost in tears.
Injured people have been ferried away from the scene of the blast
"First we saw a yellow flash, then white like lightning and then dust everywhere," said another survivor clearly still very shaken.
"We can't say what happened but it was a huge bang."
The American troops moved in quickly after the explosions as helicopters hovered overhead.
They eventually managed to seal off the UN compound, which is still known as Canal Hotel (reflecting its former function), and also closed the main road in front of the compound.
The casualties slowly emerged from the building. Some were taken to Baghdad's poorly equipped hospital in ambulances and private vehicles, others were evacuated by American helicopters.
The American forces brought in heavy machinery to dig through the rubble to find survivors and said they would continue the search for as long as it took.
The UN's humanitarian work inside Iraq was being co-ordinated from the headquarters, which also houses other UN agencies such as the World Food Programme.
It also housed the office of the UN Special Representative to Iraq, Sergio Vieira de Mello, who was working on political issues such as human rights and the restoration of democracy.
The explosion was directly underneath the second floor office of Mr Vieira de Mello. He was initially reported to be injured and trapped under the rubble, but it was later announced by the UN that he was dead.
The blast took place in the late afternoon, always a busy time for the UN. A press conference on landmines was under way, attended by journalists who caught the scenes of chaos and panic on film.
It is likely that many other visitors from aid agencies and different organisations were in the building at the time.
Ordinary Iraqis in Baghdad have been shocked by the attack.
The UN is widely seen as a positive force in Iraq which helps the people by restoring basic services such as power, water, sanitation, by equipping hospitals and schools and representing their views to the American led coalition.
Some Iraqis said they were worried that such crucial work would now be scaled down.
The UN compound was guarded by American troops. The wall outside had already been raised as a security precaution. The UN would only allow their own vehicles inside the compound and carried out extensive checks on all visitors.
However that was little protection against what appears to be a suicide bombing.
This bomb was clearly not just an attack on the UN. It was also meant to show that the American forces are still unable to guarantee the safety and security Iraqis so badly crave.