BAGHDAD - Two suicide bombers wearing vests full of explosives blew themselves up in separate attacks on Thursday, killing 76 people, including many Iranian pilgrims, in what appeared to be Iraq's bloodiest day in over a year.

The blasts occurred as apprehension grows in Iraq ahead of a pullout by U.S. troops from city centers in June, and after warnings from officials that insurgent groups may try to take advantage of that to launch attacks.

A year-end election also threatens to stir a resurgence in violence just as the sectarian bloodshed and insurgency triggered by the 2003 U.S.-led invasion appeared to be receding.

Shortly after the two attacks, the authorities in Baghdad said they had arrested the purported leader of an al Qaeda-affiliated insurgent group, Abu Omar al-Baghdadi. His arrest, which has been reported before, could not be confirmed.

One of the attacks occurred near Muqdadiya, 80 km (50 miles) northeast of Baghdad, in the volatile province of Diyala. The suicide bomber appeared to have targeted a group of Iranian pilgrims in a crowded roadside restaurant at lunchtime.

All but two of the 48 dead were Iranians visiting Shi'ite Muslim religious sites in Iraq, police said. Seventy-seven people were wounded.

It was the single deadliest attack since 50 people were killed by a suicide bomber in a restaurant near the northern city of Kirkuk on December 11 last year.

I just left the hospital of Baquba. The scenes there are catastrophic, said Abdulnasir al-Muntasirbillah, who marked his first day in office as Diyala governor on Thursday. Words can't express it. It is a dirty, cowardly terrorist act.

The blast in central Baghdad took place as a group of Iraqi national police were distributing relief supplies to families driven from their homes at the height of the violence.

Twenty-eight people died, and 50 were wounded, police said. At least five children were among the dead, they added. Some witnesses said the bomber might have been a woman.

Red Crescent food parcels, police helmets and shattered packets of chocolate biscuits were strewn in the blood pooled on the pavement after the attack, while a woman dressed in a black abaya robe wailed and beat her thighs in anguish.

It is a suicide bomber. Obviously that has the fingerprints of al Qaeda, said Baghdad security spokesman Major-General Qassim Moussawi.

BAGHDADI ARREST

Violence across Iraq has fallen sharply over the past year, but insurgents such as Sunni Islamist al Qaeda still carry out attacks. Suicide bombings are often associated with al Qaeda.

Yet, while the bombings remain routine, it has been a while since so many people were killed on a single day.

On June 17 last year, a truck bomb in Baghdad killed 63, two bombs on March 6, 2008, killed 68 people, also in Baghdad, and female suicide bombers targeting a pet market killed 99 in the capital on February 1, 2008.

Shortly after Thursday's bombings, Moussawi's office reported that Baghdadi had been arrested in east Baghdad.

Baghdadi is said to be the head of the Islamic State of Iraq, one of the groups thought to be behind suicide bombings.

We certainly hope that it's true, Pentagon spokesman Bryan Whitman said in Washington. This would be a significant capture if the report is true.

Security experts have speculated that Baghdadi was a character invented by extremists rather than a real person, but Whitman said the U.S. military did believe there was a single al Qaeda leader with that name.

Some Iraqis expect violence to increase in Iraq as rival political and armed groups position themselves ahead of a national election due to take place at the end of the year.

Iraqi officials say al Qaeda and others are also likely to try to test Iraqi security forces as U.S. troops prepare to pull out of cities ahead of a full withdrawal by the end of 2011.

Iraq