The bomber blew himself up while being patted down by policemen near the Imam Ali mosque in the old part of the holy city of Najaf, said Dr Munthir al-Ithari, the head of Najaf's health directorate.
Witnesses and officials had earlier reported two bombs explosions. Al-Ithari said a second bomb did go off, but in a different part of the town. It was not clear if there were any casualties there, the AP reports.
He said 30 deaths and 60 injuries have been recorded so far, and the toll is likely to go up. Among the dead are 22 men including five policemen.
The bombing occurred at about 10.30 am in a market in front of a religious place of worship.
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