Four American security guards were among the victims of the huge blast, which also left 33 wounded, including several schoolgirls whose bus was caught up in the massive explosion.

"According to the latest toll, there are 13 Iraqi civilians killed and four foreigners whose bodies were completely charred," an interior ministry official said on condition of anonymity.

"According to the badges we found, they are Americans," said medic Khaled Ibrahim. A US officer on the scene confirmed to AFP that the four were US citizens.

A car bomb parked on Baghdad's busy Tahrir Square went off at 11:05 am (0705 GMT) and struck a convoy of sports utility vehicles, of a sort widely used to escort prominent Iraqis or foreigners.

The explosion shook central Baghdad and sent a huge mushroom of smoke billowing into the sky. It was so powerful it sent two civilian cars flying over a wall. The vehicles came crashing down at the entrance of a tunnel.

"They were not heading down into the tunnel but the force of the explosion propelled them over the wall," said a witness.

The bodies of the four foreigners, presumed to be security guards, were completely charred inside the wreckage of their vehicles.

"True it was a US target, but what about civilians? They are innocent. We helped rescue about 15 schoolgirls, three of them were badly wounded," said witness Ahmed Ghanem Yehya.

There was no immediate claim of responsibility for the blast, the latest in a string of car bombs and other insurgent attacks that have killed some 300 people already this month.

Shots were fired as chaos filled the busy square, which commands access to the Jumhuriya bridge leading to the fortified Green Zone administrative enclave.

Police and foreign security guards secured the area, as ambulances rushed the wounded to nearby hospitals. A man could be seen trying to crack open the door of a damaged vehicle with a sledgehammer to extract its occupant.