Witnesses said at least one of the first two bombers slammed a car into defences around the concrete towers of the Palestine and Sheraton hotels, blasting a path for a third in a cement truck on Monday.
It exploded beside a US Bradley armoured vehicle on guard duty, sending up a huge ball of flames and smoke.
There were no initial reports of US military casualties, a US military spokesman said. Recent media reports have focused attention on the fact that the US toll in Iraq is just three short of the headline figure of 2000.
The explosions were captured in television footage by cameras trained on the area after the first blast and perfectly placed for subsequent detonations.
There was widespread superficial damage in the Palestine hotel, and windows and doors were shattered as dust filled buildings nearby; one 2kg piece of blackened, jagged metal the size of a dinner plate landed, among many smaller pieces, outside the Reuters bureau 500 metres away.
The blasts scattered debris
across the city centre
At least 17 people were killed and 11 wounded in the blasts, police said.
Iraq had been relatively calm despite expectations that attacks would be stepped up during a referendum on a US-backed constitution and the trial of ousted president Saddam Hussein last week.
Two Sunni Arab provinces have returned resounding no votes on the charter, officials confirmed on Monday, and its fate could rest with voters in a volatile region in the north.
Iraq's electoral commission head Abdul Husain al-Hindawi told reporters on Monday that in the Sunni Arab province of al-Anbar, heartland of the anti-US uprising, 96% voted no according to preliminary results.
The sentiment was the same in Salah al-Din province. Voters there registered an 81% rejection of the charter. Final results are expected within the next two days.
Nineveh, with Iraq's third-largest city Mosul as its capital, is seen as a swing province that could determine the fate of the charter, which appears to have deepened sectarian divisions.
Under Iraqi law, the draft constitution will be struck down if two-thirds of voters in three provinces reject it.
Shia and Kurdish leaders, who drew up the charter, are hoping it will unite Iraq and defuse violence aimed at toppling the US-backed government.
Leaders of the Sunni Arab minority, once dominant under Saddam, are opposed to the constitution, fearing it will give Shia and Kurds too much power and oil resources.
Although violence eased during the 15 October referendum and the opening of the trial of Saddam four days later, the results of the poll threaten to boost sectarian tensions that have raised fears of civil war.
US marine killed
A US marine was killed by small arms fire during combat operations in Ramadi on Sunday, the military said on Monday.
That raised the number of US military deaths in Iraq since the 2003 US-led invasion to 1997.
US-led forces have failed to stem
the violence in Iraq
In other news, two Moroccan embassy employees working in Baghdad have been missing since Thursday, the authorities said on Monday.
Driver Abd al-Rahim Bualam and assistant Abd al-Karim al-Muhafidim, both working at the consulate in Baghdad, went missing after returning by road from a brief trip to Jordan, the official Moroccan Maghreb Arabe Presse news agency said, citing a Foreign Ministry statement.
The government was working with the Iraqi authorities to find the missing employees, the statement said.
Also on Monday, a bomb attack against the ultra-secure Baiji petroleum refinery killed five participants at a meeting attended by Americans and injured several others, police said.
The bombs were placed against walls surrounding a residential complex at the base, 200km north of Baghdad, where the meeting was taking place between refinery managers and the Americans, according to police Colonel Hasan Salih.
He said the bodies of five Iraqis were recovered from the collapsed ruins of the building where the meeting was being held.
Salih said nine Iraqis were injured. There was no indication whether any US nationals were among the casualties.