The attack is the latest in a recent series of major bomb attacks on civilians
At least 13 people have been killed and dozens wounded in a bomb attack in the Iraqi capital, Baghdad.
The bomb, planted on a motorbike at a motorcycle market in the city centre, was the latest in a recent series of major attacks on civilian targets.
On Wednesday some 70 people were killed in the Sadr City area of Baghdad.
American troops are scheduled to pull out of all Iraqi cities by next Tuesday. Officials say the attacks will not affect their plans.
The motorbike market, which only takes place on Fridays, has been targeted by bombers in the past.
"People burned and bodies were thrown everywhere," said witness Omar Hashem, who was covered in blood.
"At first we ran away but we returned to help the victims," he told the AFP news agency.
Maytham Abdelamir, a local trader, also told AFP: "There was a huge shockwave. I saw so many people burned. Others were thrown in the air. My friend was killed."
RECENT IRAQ BOMBINGS
24 Jun: 70 dead, Baghdad
20 Jun: 60 dead, Kirkuk
10 Jun: 20 dead, near Nasiriyah
On Thursday, Iraqi Prime Minister Nouri al-Maliki - speaking in the wake of recent bombings - tried to reassure the Iraqi people that his forces could protect them.
He said the attacks were part of a plan to "awaken sectarianism and create chaos, and prevent the Iraqi people from standing on their own feet".
But correspondents say the latest bombings are raising questions about the ability of Iraqi security forces to combat a determined insurgency when the Americans pull back.
The BBC's Jim Muir, in Baghdad, says residents of Sadr City are angry, and accuse the Iraqi security forces of not doing enough to protect them as US troops complete their withdrawal.
The US troop pullout from urban areas is a significant element of an agreement between Washington and the Iraqi government, under which all US soldiers are due to leave Iraq by 2012.