Police said Harith al-Obeidi, head of the biggest Sunni bloc in the Iraqi parliament, was shot at close range by what appeared to be a teenage gunman after Friday prayers. The shooting happened at the al-Shawaf mosque in the Yarmouk neighborhood of western Baghdad. At least three others were killed, and several more people wounded by gunfire and what appeared to be a grenade attack. The gunman was later shot dead by mosque guards.
Iraqi president Jalal Talabani called for national unity "in the face of powers that seek to provoke strife and destabilize the country." Prime Minister Nouri al-Malaki pledged a high-level investigation and called it a failed attempt to plant sectarian discord.
A spokesman for the Sunni party itself, though, blamed Al Qaeda in Iraq (AQI) for the assassination. AQI considers Iraqi lawmakers part of a puppet government controlled by the United States, and in the past has claimed responsibility for killing politicians as well as attacking Iraqi security forces.
Mr. Obeidi, deputy head of parliament's human rights committee, had recently become the leader of the Accordance bloc after the previous chief, Ayad al-Samarai, was elected speaker of parliament.
Mr. Maliki warned Iraqi military leaders Thursday that violence would likely increase as insurgents tried to set back security gains around the June 30 deadline for US troops to withdraw from the cities.
Violence has decreased dramatically from a year ago, but suicide bombings over the past week have killed dozens of people. Most of them have targeted Shiite areas, sparking concern that there could be retaliation against Sunni groups by Shiite militias that would launch another cycle of sectarian violence.
Obeidi, in his sermon, criticized the continued detention of Iraqis without charge or access to lawyers. Many of the detainees are Sunni.
The Accordance bloc, and its main group, the Iraqi Islamic Party, had split with Mr. Maliki's Shiite-led coalition, but later rejoined it. The bloc is the biggest Sunni faction in parliament. The participation of Sunni Arabs in a political process they have largely boycotted in the past is seen as crucial to Iraqi stability.
If Sunni insurgents, rather than Shiite extremists, were responsible for Obeidi's killing, it would be less likely to ignite a renewal of the sectarian violence that flared into civil war three years ago.
The attack, though, in a mosque during the Muslim holy day, is an indication of Iraq's still-fragile security and the persistent ability of insurgent groups to carry out high-profile assassinations.