BAGHDAD  - Four car bombs exploded in Baghdad on Tuesday, killing at least 13 people and wounding 75, underlining a political crisis that threatens to revive sectarian strife in Iraq.

The first blast hit a group of day labourers gathering for jobs in the poor northeastern Sadr City area of the capital, leaving a chaotic scene of scattered shoes and food, and pools of blood. The bomb killed at least eight people and wounded 24, police and hospital sources said.

"We were all standing waiting to earn our living and all of a sudden it was like a black storm and I felt myself thrown on the ground," said Ahmed Ali, a 40-year-old labourer whose face and hair were burned by the explosion. "I fainted for a while then I woke up and hurried to one of the cars to take me to the hospital," said Ali, lying on a bed in the emergency room at Imam Ali hospital in Sadr City.

The second blast near a traffic roundabout in Sadr City killed two people and wounded 26 others, the sources said.

Two other car bombs exploded in mainly Shia northwestern areas of Baghdad, killing three people and wounding 25, sources said. One car blew up near two schools in the Shula district, the other on a busy commercial street in Hurriya.

Violence in Iraq has dropped sharply from the height of sectarian killing in 2006-07, but insurgents and militias still carry out daily attacks and assassinations in an attempt to undermine the government.

Iraq has been hit by a series of bombings targeting Shias during the worst political crisis in a year, which threatens to break up a fragile coalition government and has raised fears of renewed sectarian violence after US troops left on December 18.

The government of Shia Prime Minister Nuri al-Maliki moved last month to arrest Sunni Muslim Vice President Tareq al-Hashemi on charges he ran a death squad and then sought to sideline a Sunni deputy prime minister after he branded Maliki a dictator.

Hashemi denied the charges and sought refuge in Iraq's semi-autonomous Kurdish region, where he is unlikely to be arrested.