BAGHDAD, Iraq – A terrorist strapped with more than 100 pounds of explosives and disguised in an army uniform blew himself up in a crowded mess hall yesterday as brutal attacks across Iraq killed more than 50 people. In a dramatic raid, Iraqi and American forces freed an Australian hostage, Douglas Wood, who was held by insurgents for 47 days.

The swell in violence appeared aimed at derailing stepped-up efforts by Shiite politicians to bring the disaffected Sunni Arab minority into the political process. The two groups are trying to write a groundbreaking constitution and gain necessary approval to allow government-building to move forward.

Al Qaeda in Iraq claimed responsibility for the mess-hall bombing at an army base in Khalis, about 45 miles north of Baghdad. The attacker “was invited to this lunch, and we ask Allah that he finishes his food in paradise,” Al Qaeda said in a statement posted on a terrorist Web site.

The terrorist, wearing an army uniform and strapped with about 110 pounds of explosives, waited until soldiers gathered for lunch before blowing himself up, Iraqi army Colonel Saleh Al Obeidi said. The blast killed 26 soldiers and wounded another 26 belonging to Al-Salam battalion of the 2nd Brigade of the Iraqi army in Diyala province, the army said.

“I couldn’t recognize one body from another. Body parts were scattered everywhere, because it was a huge blast,” Mr. Al Obeidi said. “It is a fortified base, and we never believed any incident would occur inside this base.”

The man was among a group of construction workers hired to expand the mess hall’s kitchen and “that’s why he was able to go this far,” Mr. Al Obeidi said.

“The attacker picked the right time to carry out the attack, when two army groups were supposed to be inside for lunch. He blew himself up as soon as he entered the hall, avoiding all the soldiers so they would not recognize that he was a stranger,” Mr. Al Obeidi said.

The explosives allegedly were brought onto the base Tuesday inside several trucks carrying construction materials, he said.

Mr. Wood, dressed in traditional Arabic garb, was found huddled beneath a blanket, and residents of the home where he was insisted he was their ailing father. But troops, who were acting on a tip, would not be deterred.

Mr. Wood was haggard but gave cameras a thumbs-up during a medical checkup at an American military facility.

“He has been blindfolded, handcuffed, he has not been well looked after,” Australia’s counterterrorism chief, Nick Warner, said after Mr. Wood was freed from a house in one of Baghdad’s most dangerous neighborhoods.

Also yesterday in eastern Baghdad, a suicide car bomber slammed into two police cars on patrol, killing 10 people, including eight police officers, and injured another 23. The suicide bomber drove a German luxury car into the police cars as they were stopped at an intersection, police said.

Three mortar shells fell on Abu Ali Restaurant, a well-known Baghdad kebab eatery, killing five Iraqis and wounding eight, police said. A nearby police headquarters was the apparent target, police said. Insurgents kidnapped and killed two senior officers in Kirkuk’s anti-terrorist squad, along with their driver and the son of one of the men. The bodies of all four were found dumped street side in the northern city with gunshots to their heads.

The tribunal that will put Saddam Hussein on trial released its second videotape in a week yesterday, this one showing an investigating judge questioning three senior officials of the former dictator’s regime – including Saddam’s half brother Sabawi Ibrahim.

Iraqi legislators, meanwhile, seemed close to agreement on a demand by Sunni Arabs for more participation in the effort to draft a constitution.

A Shiite-dominated parliamentary committee drafting Iraq’s new constitution offered a compromise to the country’s Sunni Arab minority in an effort to break a deadlock over demands they have a bigger say in drawing up the charter. The offer suggested that 13 additional Sunni Arabs join the committee in a parallel body.

The Sunni Arab community has said it wants 25 more people to join their two legislators already on the committee, and the two Sunni Arab representatives rejected the offer. But representatives from the Sunni Arab community will meet today to discuss the proposal.