At least 20 people have been killed and more than 60 injured in a suicide car bombing outside the US headquarters in the Iraqi capital.
The blast went off as Iraqis waited for appointments
The huge blast occurred at about 0800 (0500GMT) near Assassin's Gate - a heavily fortified entrance to one of Saddam Hussein's former palaces.
Most victims are thought to have been Iraqis waiting to meet US officials.
It is the deadliest attack since New Year's Eve and comes as the US tries to persuade the UN to return to Iraq.
The US military is seeking clarification about whether two of those killed in the latest attack were American nationals.
The chief US administrator in Iraq, Paul Bremer, said the attack was "an outrage - another clear indication of the murderous and cynical intent of terrorists to undermine freedom, democracy and progress in Iraq."
"They will not succeed," he added in a statement.
On Sunday evening, at least 13 people were injured in a bomb attack in the Shia holy city of Karbala.
The blast happened in a busy street 2km away from the Shrine of Abbas, an important place of pilgrimage.
It was the latest in a series of attacks on Iraq's majority Shia community, but it is not clear who was responsible.
Force of the blast
The Baghdad blast happened as Iraqis on business or seeking work were queuing to enter the complex on what is a normal working day in Baghdad.
The compound - the Republican Palace in Saddam Hussein's time - is now the civilian and military headquarters of the US administration.
One thousand pounds (500 kg) of explosives carried in a white Toyota pick-up detonated outside the main entrance to the complex, said US military spokesman Colonel Ralph Baker.
MAJOR ATTACKS SINCE 1 MAY
18 January 2004: 18 reported killed outside coalition HQ, Baghdad
31 December 2003: Eight killed in Baghdad restaurant blast
14 December: Car bomb at police station kills 17 in Khalidiya, west of Baghdad
12 November: 26 die in suicide attack on Italian police base in Nasiriya
2 November: 16 US soldiers die as Chinook helicopter downed
27 Oct: Red Cross and other buildings in Baghdad bombed, more than 30 killed
29 Aug: Mosque near Najaf bombed, at least 80 dead including Shia Islam's top cleric in Iraq
19 Aug: UN headquarters in Baghdad bombed, 23 killed including head of mission
List covers attacks since US declared war effectively over
Six Americans, including two soldiers, were reported to be among the injured.
Most of those who died were inside their cars when the fireball erupted, one coalition official said. "They didn't have a chance", he said.
Iraqi police announced through loudspeakers that a reward would be given to anyone providing information on who carried out the attack.
The last big car bomb attack in Baghdad was on New Year's Eve, when eight people were killed when a bomb exploded outside a busy restaurant in the city centre.
The blast comes a day after a bomb attack on a patrol north of Baghdad took the number of American troops killed since the war began in Iraq in March last year to 500.
Meanwhile as debate continues over the issue of how Iraq should be run in future, the US says it intends to ask the United Nations to back its plans for the transfer of power.
Mr Bremer is to meet UN Secretary-General Kofi Annan on Monday in a drive to get the UN to return to Iraq.
He is also expected to seek UN help in convincing the Shia majority that US proposals for an unelected transitional government are the best way to meet a July deadline for a handover of sovereignty.