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It was not immediately clear how many people were killed or wounded in the attack, which occurred at about 7:30 am (0430 GMT) in the Bab al-Sharji area. Ambulances could be heard rushing to the scene as helicopters flew overhead.
An interior ministry official, speaking on condition of anonymity, said the explosion was the work of a suicide car bomber. State broadcaster Al- Iraqiya TV also reported this as the cause of the attack.
The blast came after violence across Baghdad on Thursday killed 60 people.