A rapid succession of co-ordinated bomb and mortar attacks in central Baghdad targeting government ministries has left at least 100 dead and 600 wounded.
The attacks make it the bloodiest day in the Iraqi capital since United States troops withdrew from urban centres in June.
The United States has condemned the attacks, but says its soldiers are limited in what they can do to help. American ambassador to Iraq Christopher Hill says US soldiers cannot step in to help unless asked because of the agreement handing over security to Iraqi forces at the end of June.
Six explosions struck within minutes of each other.
One blast was close enough to Iraq's parliament - in the heavily guarded government and diplomatic area known as the Green Zone - to shatter some windows.
Other blasts were near the Finance Ministry and Baghdad's central governorate building, causing huge destruction.
Police say vehicles packed with explosives exploded outside two government ministries, and a short time later mortar bombs fell on other areas of Baghdad.
Iraqi forces took over responsibility for security in the city in late June, and since then most attacks have affected poor neighbourhoods rather than government or diplomatic targets.
US plans to withdraw forces from Iraq over the next two years remain in place despite the wave of bombings, a Pentagon spokesman says.
"These are attempts by insurgent groups to try to exploit sectarian tensions," he said.
Iraqis have pointed the finger at their domestic security forces, which in turn blamed members of executed former dictator Saddam Hussein's regime.
But an adviser to the Iraqi government, Saad Yousef Al-Muttalabi, has accused Saudi-based religious institutions of sponsoring the co-ordinated attack.
Iraqi Prime Minister Nuri al-Maliki has ordered a review of security.