Iraqi security officials said parliament lawmaker Khalid al-Fahdawi, a Sunni, was among the dead in the 9:40 p.m attack.
Maj. Gen. Qassim al-Moussawi, a spokesman for Baghdad's military operations command, confirmed the bombing happened inside the Um al-Qura mosque during prayers in the western Baghdad neighbourhood of al-Jamiaah. The blue-domed building is the largest Sunni mosque in Baghdad.
That the bomber detonated his explosives vest inside the mosque is particularly alarming, as it is reminiscent of a 2006 attack on a Shia shrine in the Sunni city of Samarra that fueled widespread sectarian violence and nearly ignited a nationwide civil war. In that strike, Sunni militants planted bombs around the Samarra shrine, destroying its signature gold dome and badly damaging the rest of the structure.
No group immediately claimed responsibility for Sunday's bombing, but suicide attacks generally are a hallmark of al-Qaeda, which is dominated by Sunnis. Intelligence officials have speculated that al-Qaeda will do almost anything to re-ignite sectarian violence, but the group recently had focused on attacking Iraqi security forces and the government to prove how unstable Iraq remains.
Two security officials and medics at two Baghdad hospitals put the casualty toll at 29 dead and 38 wounded. All spoke on condition of anonymity because they were not authorized to release the information.
Al-Moussawi put the death toll at only six and said there was no significant damage to the mosque. Conflicting death tolls are common immediately after attacks in Iraq.
In a statement early Monday, Prime Minister Nouri al-Maliki called on Iraqis to stand strong against terrorists and "pursue them wherever they are."
"Solidarity and unity, and standing as one line behind the army and the police, are the only way to eliminate this danger, which does not differentiate between the Iraqis and targets all of us," al-Maliki said.
The attack hit Sunnis who were praying in a special service during the holy Muslim month of Ramadan, which ends Tuesday. It demonstrates anew that security measures to protect Iraqis as U.S. forces prepare to leave remain riddled with gaps, and shows the extent to which militants want to extend violence even as the eight-year U.S. presence winds down.