More than 150 people have been killed and hundreds injured in a series of bomb attacks and shootings across Iraq.
In the worst incident, at least 112 people were killed and some 160 injured when a car bomb exploded in Baghdad's mainly Shia district of Kadhimiya.
During the night, gunmen killed 17 men in the nearby town of Taji after dragging them from their homes.
Al-Qaeda in Iraq claimed it had begun a nationwide bombing campaign to avenge a recent major offensive on rebels.
In a statement on a website, the group said it acted after US and Iraqi forces attacked insurgents in the northern town of Talafar.
In a separate development the leader of al-Qaeda in Iraq, Abu Musab al-Zarqawi, purportedly declared "war against Shias in all of Iraq" in an audio tape released on the internet.
Wednesday became one of the deadliest days in Iraq since the US-led invasion in March 2003.
BLOODIEST VIOLENCE IN IRAQ
28 Aug 2003 - 85 dead
Car bomb at Najaf shrine kills Shia cleric Muhammad Baqr Hakim and many others
1 Feb 2004 - 105 dead
Twin attacks on Kurdish parties' offices in Irbil
2 March 2004 - 140 dead
Suicide bombers attack Shia festival-goers in Karbala and Baghdad
24 June 2004 - 100 dead
Co-ordinated blasts in Mosul and four other cities
28 Feb 2005 - 114 dead
Suicide car bomb hits government jobseekers in Hillah
16 Aug 2005 - 90 dead
Suicide bomber detonates fuel tanker in Musayyib
The attacks once again showed the limits of military might as a solution to the insurgency, the BBC's Rob Watson says.
When under military pressure in one part of Iraq, the insurgents simply strike elsewhere, our defence and security correspondent says.
The attacks came as a final draft of the Iraqi constitution was handed to the UN.
After months of negotiations, the draft is due to be distributed to Iraqis before a referendum on it on 15 October.
In attacks elsewhere:
- Six Sunnis are shot dead in Taji in an apparent revenge attack after the killings of the 17 men, who are thought to be Shias, police say
- Three Iraqi soldiers are killed when a car bomb targets their patrol in the Al-Adel district of western Baghdad
- A bomb explodes by an Iraqi National Guard convoy in the northern Baghdad district of Shula, killing at least four people and wounding 22
- Two US soldiers are wounded as a suicide bomber rams a vehicle packed with explosives into their Humvee in east Baghdad
- Gunmen kill four policemen in Baghdad's northern district of Adhamiya. Three Iraqi soldiers and four policemen die when a suicide car bomber strikes as rescuers arrive to help
- A suicide car bomber attacks a US convoy close to the Rashid Hotel in the Green Zone, although it is not yet known if there were any other casualties
- A suicide bomber blows himself up in Baghdad without causing any other casualties.
A suicide bomber drove his car at 0630 (0230 GMT) into queues of construction workers who had gathered on Oruba Square in Kadhimiya, Iraqi police spokesman Maj Musa Abdel Kerim said.
According to some reports the attacker lured the workers towards the vehicle before detonating the bomb.
"We gathered and suddenly a car blew up and turned the area into fire and dust and darkness," one of the workers, a man named Hadi, told Reuters news agency.
British Foreign Office Minister Kim Howells said in Baghdad that the latest attacks "will not stop the heroic attempts of the Iraqi people to create a safe, brighter future".
There have been frequent sectarian killings in Baghdad and central Iraq as mainly Sunni insurgents seek to incite fear and hatred between the Muslim communities.