Aljazeera's correspondent in Baghdad said that the casualty figures from Wednesday's blast are likely to rise.
The explosion appeared to have taken place where cars queue to drop off and pick up visitors to the Green Zone, a huge, heavily defended complex in central Baghdad.
A Reuters correspondent reported seeing a large plume of black smoke rise into the air. A man walked away from the blast scene with blood oozing from a chest wound.
A US military spokesman said he had no information about the incident.
"We were gathering outside the convention centre seeking jobs," said one witness, Alla Hassan. "We were thrown on the ground. Then I saw many dead people on the ground."
The heavily guarded Green Zone
is a frequent target of attack
French news agency AFP quoted an Iraqi policeman saying that it was a car bomb that exploded and left many casualties.
"It was a car bomb. It happened at 09:20 (05:30 GMT) by the front gate where cars go to enter the Governing Council building," the policeman said.
"There were many casualties," he added.
Resistance fighters frequently fire mortars and rockets towards the zone, and car bombs have been detonated at entrances to the area. In May, the then head of Iraq's interim government was killed in a car bomb attack as he entered the zone.