A riot erupted in a temporary detention centre in central Baghdad on Thursday, an Interior Ministry source said, and authorities called for back-up to help calm the situation.
Around 13 ambulances and about 30 anti-riot vehicles carrying fully equipped police entered the Rusafa detention centre in western Baghdad, reports said.
The reporter said other police and army vehicles had also surrounded the centre, which is a holding facility for people who have not yet been charged with crimes or for those who have undergone trial and are to be released from prison.
"A group of prisoners were rioting. They burned tents. This is not the first time it has happened but this time the issue is major," a security guard said as he was leaving the centre.
"The centre includes all the criminals and terrorists from al Qaeda group, whose hands are stained with the blood of Iraqis," the guard said on condition of anonymity.
No casualties had so far been reported.
Another policeman at a checkpoint leading to the centre said things were "upside down" inside the centre but declined to give further details.
Earlier this month, two inmates were killed and seven policemen and seven inmates wounded in a prison riot in Tikrit in Iraq's northern Salahuddin province.