Three powerful explosions have killed at least 17 people outside Baghdad hotels used by foreign media and contractors, police say.
Suicide bombers are believed to have driven vehicles into barriers outside the Palestine and Sheraton hotels where the attacks were caught on camera.
A BBC correspondent says the bombers appear to have chosen the hotels to ensure maximum publicity.
The blasts come amid speculation the poll on the constitution may fail.
One source told AFP news agency the dead were mainly security guards, hotel employees and passers-by. At least nine people were injured.
A US armoured vehicle was destroyed in one of the blasts but it was empty at the time, AP news agency reports.
Iraq's national security adviser, Muwafaq al-Rubaie, told reporters the attackers had apparently intended to seize the Palestine Hotel and take journalists hostage but he did not say what evidence he had of such a plan.
UK Foreign Secretary Jack Straw condemned the attackers as "terrorists who really don't mind who they kill, provided they kill somebody in the name of a totally perverted ideology".
Police said the co-ordinated attacks were carried out by three suicide vehicle bombers and happened inside a matter of minutes.
The first two rammed holes in the security wall around the hotels.
The third bomber, driving a cement lorry laden with explosives, tried to penetrate one of the breaches but got stuck, came under fire and set off his charge.
The BBC's Jim Muir describes the first blast as a huge explosion which shook the whole of central Baghdad.
The second massive blast was followed by bursts of gunfire.
But the third blast was the most dramatic, our correspondent says: a vast flash of orange, followed by an enormous explosion which sent a massive cloud into the sky, shook buildings and scattered debris far around.
The attacks occurred just before sunset when Muslims break their fast during their holy month of Ramadan and many residents were on their way home to eat.
The two hotels make natural targets for the insurgents as they are used by foreign contractors and journalists, says the BBC's defence correspondent, Rob Watson.
The insurgents are well aware that, with so many bombs every day, it takes something spectacular to gain the attention of the international media, our correspondent says.
Hanging in the balance
Two Sunni-dominated provinces in Iraq have rejected the draft constitution, according to partial results given by election officials.
The document will fail if three out of the 18 provinces vote "No" by two-thirds or more and all eyes are now on Nineveh, where the result is due to be announced within two days.
In other violence reported on Monday:
- Gunmen shoot dead 12 builders, including five brothers, in an execution-style attack near Musayyib, outside Baghdad
- Baghdad police report finding seven bodies, three of them female, dumped in and around the city
- A suicide car bomb attack near an Iraqi police patrol in Baghdad's north-eastern district of Shaab kills two.