BAGHDAD - The head of Iraq's biggest Sunni Muslim bloc in parliament was shot dead at a mosque after delivering a sermon yesterday, underlining fears that violence might mount before a deadline in two weeks for U.S. forces to withdraw from Iraqi cities.

Politicians said the assassination, in a neighborhood once one of Baghdad's most dangerous, was an attempt to reignite sectarian tension that has diminished but persists in Baghdad and other parts of Iraq.

The killing comes as politicians have begun sometimes tense deliberations over new coalitions ahead of parliamentary elections in January.

Prime Minister Nouri al-Maliki warned Thursday at a gathering of hundreds of top military commanders from across the country that "terrorist operations" could increase ahead of the U.S. withdrawal from cities and the elections.

The slain man, Hareth al-Obaidi, 45, who headed the human rights committee in parliament, was an outspoken critic of human-rights abuse by security forces inside Iraqi jails. He became head of the Sunni bloc in parliament in May after his predecessor was chosen speaker.

"This shows that the security situation is still fragile and dangerous," said Saleem al-Jubouri, a spokesman for the Sunni coalition known as the Iraqi Accordance Front, which Obaidi headed. "The government must provide more security."

Obaidi is the third lawmaker - and the first Sunni lawmaker - to be assassinated since parliament was elected in 2005, when many Sunnis boycotted the vote.

Shortly after noon, a gunman in civilian clothes, who witnesses and police estimated was 14 or 15 years old, shot Obaidi twice in the back of the head, killing him instantly.

Obaidi had just finished delivering a sermon, as he does every Friday at Al-Shawaf mosque in the neighborhood of Yarmouk, in western Baghdad.

In the sermon, he called on authorities to investigate widespread allegations of torture in Iraqi prisons.

And in a session for his committee Thursday in parliament, the lawmaker had announced plans to summon security officials and ministers before the legislature to answer questions in connection with human-rights abuses.

After firing his weapon, the gunman threw a hand grenade at the crowd, killing three other people and injuring 11. Witnesses said he then tried to escape through a back door of the mosque but was chased down and killed by guards.

Witnesses said they saw worshipers running out of the mosque shortly after they heard gunshots.

Some were crying; others were in panic.

Merchants shuttered their stores.

Police cordoned off the area, preventing anyone from approaching the mosque, where witnesses said Obaidi's body remained for a few hours after the killing.

Many in the neighborhood questioned how the assailant could evade tough security measures - from checkpoints in nearby streets to searches at the mosque's entrance - and smuggle a gun and grenade into the hall packed with worshipers.

The slain lawmaker's brother, Mohammad al-Obaidi, said he believed the guards had helped the gunman enter the mosque. "It was an organized crime," he said, sobbing.

Obaidi was also a university professor with a doctorate in Islamic studies. He had two wives and seven children.