"Iraq is still under foreign occupation and Iraqis continue to die in great numbers"
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A hospital official confirmed the deaths and said that 20 more were wounded, among them six policemen.
The overwhelmingly Shia city has been the scene of inter-Shia rivalry between the al-Mahdi army, the Supreme Islamic Iraqi Council and the Fadhila party.
The security forces, especially the police, have been widely infiltrated by the Shia groups whose rivalry over control of southern Iraq's largest city has escalated since British forces were withdrawn in early September.
Also on Tuesday, police in Baquba, which lies north of Baghdad, revised the toll from Monday's suicide attack on a village mosque.
Death toll rises
Brigadier General Khaider al-Timimi, a police official, said: "We have a total of 28 people killed and 34 wounded."
The bombing had targeted a Shia-Sunni reconciliation meeting at a mosque.
The meeting was between the Shia al-Mahdi Army and the Sunni armed group known as the Brigades of the 1920 Revolution, an Iraqi security official said.
Meanwhile, Tuesday's double car bombing in Baghdad occurred at mid-morning outside al-Rafidain bank in Zayunah, a mixed neighbourhood.
"The blasts occurred 30 seconds apart," a security official said.