Kurds in northern Iraq are in mourning after a double suicide bombing at the offices of their main political parties, that killed at least 56 people.
People had been celebrating Eid al-Adha when the offices were hit
Senior political figures were among the dead, and some 200 people were wounded, in the blasts on Sunday in Irbil.
US chief administrator in Iraq Paul Bremer condemned the "cowardly attack on innocent human beings".
He said it was also an attack on the principle of democratic pluralism in Iraq.
Communities across the region reportedly flew black flags in place of those for their two main parties - Kurdistan Democratic Party (KDP) and Patriotic Union of Kurdistan (PUK).
Kurdish television turned to sombre music. The northern region is in shock and mourning, said the BBC's World Affairs Correspondent Mike Wooldridge in Iraq.
In virtually simultaneous attacks, the bombers hit the KDP and PUK offices packed with guests for the Muslim holiday of Eid al-Adha.
Kurdish officials blamed al-Qaeda and its allies and said they feared the death toll could rise above 100.
The KDP and PUK between them largely run northern Iraq and are strong allies of the US-led coalition which ousted Saddam Hussein. Irbil hosts the Kurdish parliament.
The bombings have dealt a severe blow to the two parties and their leadership as critical negotiation is taking place ahead of the installation of a new interim Iraqi government.
Our correspondent says there is speculation that the attacks may be intended to drive a wedge between Kurdish north and the rest of Iraq.
"I want to express my outrage at today's terrorist bombings which constituted a cowardly attack at both innocent human beings as well as on the very principle of democratic pluralism in Iraq," said Mr Bremer in a statement.
Senior officials killed
Iraqi Foreign Minister Hoshiyar Zebari - a Kurd - blamed the Muslim militant group Ansar al-Islam for the attacks.
The Kurds and the United States say the group is allied with Osama Bin Laden's al-Qaeda network.
Ansar al-Islam was expelled from northern Iraq by Kurdish and coalition forces during last year's war.
RECENT MAJOR ATTACKS
1 February: At least 56 killed by twin suicide bombings during celebrations in Kurdish city of Irbil
18 January: 18 reported killed outside coalition HQ, Baghdad
14 December: Car bomb at police station kills 17 in Khalidiya, west of Baghdad
12 November: 26 die in suicide attack on Italian base in Nasiriya
But PUK spokesman Sabah Sabir told the BBC that Ansar had recently re-emerged because of the increasing volatility in Iraq.
Mohammed Ihsan, minister for human rights for the Kurdish regional government, said the dead included senior figures in the provincial government .
Among them were the Irbil Governor Akram Mintik, Deputy Prime Minister Sami Abdul Rahman, Minister of Council of Ministers Affairs Shawkat Sheik Yazdin and Agriculture Minister Saad Abdullah.
The bombers, witnesses said, made their way through checkpoints outside both venues and detonated their bomb belts once inside.
"On the first day of Eid we receive people and well-wishers and that's why security wasn't as tight as during the rest of the days," said Mohammed Ihsan.
"They [the attackers] took advantage of this."