The troops, under cover of attack helicopters, killed at least 22 insurgents in the offensive, the U.S. military said.

The thunderous explosion at the Khillani mosque in the capital's commercial area of Sinak sent smoke billowing over concrete buildings. On Sunday, officials lifted a curfew aimed at preventing retaliatory violence after last week's bombing of a Shiite mosque in Samarra.

Gunfire erupted shortly after the blast, which a police officer said went off near the Khillani mosque in the commercial area of Sinak.

A police officer, who spoke on condition of anonymity because of security concerns, said the car that exploded was parked in a lot near the mosque and it damaged the outer wall of the building.

Police and hospital officials said at least 75 people were killed and 204 were wounded, adding that the toll could rise as bodies were pulled from the debris.

The mosque's imam, Sheik Saleh al-Haidari, said it was a truck bomb and the explosion hit worshippers as they were leaving afternoon prayers.

"This attack was planned and carried out by sick souls, damaging the mosque's outer wall and collapsing my office and the room above it," al-Haidari told The Associated Press by telephone.

"There are number of bodies being pulled from the rubble and a number of worshippers were killed or injured," he said, adding that he was not inside the mosque when the blast occurred.

AP Television News video showed a huge pile of rubble where the wall used to be, but its turquoise dome was intact.

The raids, dubbed "Operation Arrowhead Ripper," took place in Baqouba, the capital of Diyala province, and involved air assaults under the cover of darkness, the military said. The operation was still in its opening stages, it said.

The commander of Iraqi military operations in Diyala, Maj. Gen. Abdul-Karim al-Rubaie, said handcuffs, swords and electricity cables — apparently used as torture implements — had been seized from militant safe houses in the area.

The operation was part of new U.S. and Iraqi attacks on Baghdad's northern and southern flanks, which military officials said were aimed at clearing out Sunni insurgents, al-Qaida fighters and Shiite militiamen who had fled the capital and Anbar during a four-month-old security operation.

A top U.S. military official said American forces were taking advantage of the arrival of the final brigade of 30,000 additional U.S. troops to open the concerted attacks.

"We are going into the areas that have been sanctuaries of al-Qaida and other extremists to take them on and weed them out, to help get the areas clear and to really take on al-Qaida," the senior official said on condition of anonymity because he was not authorized to speak about the operation. "Those are areas in the belts around Baghdad, some parts in Anbar province and specifically Diyala province."

Al-Qaida has proven to be an extremely agile foe for U.S. and Iraqi forces, as shown by its ability to transfer major operations to Baqouba from Anbar province, the sprawling desert region in western Iraq. There is no guarantee that driving the organization out of current sanctuaries would prevent it from migrating to other regions to continue the fight.

In recent months, the verdant orange and palm groves of Diyala have become one of the most fiercely contested regions in Iraq. The province is a tangle of Shiite and Sunni villages that has played into the hands of al-Qaida and allied militants who have melted into the tense region and sought to inflame existing sectarian troubles.

Al-Qaida has conducted public killings in the Baqouba main square and otherwise sought to enforce an extreme Taliban-style Islamic code. The terror organization's actions in the province have caused some Sunni militants, al-Qaida's natural allies, to turn their guns on the group with U.S. assistance and blessing. Some militant Shiites are likewise joining government forces in a bid to oust the foreign fighters and Muslim extremists.

Separately, the U.S. military announced the death of an American soldier in Baghdad. The soldier was killed by small arms fire during combat in an eastern section of the capital, a military statement said. No other soldiers were wounded in the attack, which took place Monday, it said.

The death brought to at least 3,528 the number of U.S. military personnel who have died since the beginning of the Iraq war in March 2003, according to an AP count. The figure includes seven military civilians. At least 2,889 died as a result of hostile action, according to the military's numbers.

In southern Iraq, police and hospital officials said the death toll reached 30 in clashes that continued into a second day between Mahdi Army fighters and Iraqi security forces in Nasiriyah, about 200 miles southeast of Baghdad.

Some 150 people were wounded, authorities said. The officials, who declined to be identified because they feared retribution, said most of the casualties were police officers or militiamen.

Nine mortar shells were launched early Tuesday at police headquarters in the town, and three policemen were injured, police said.

Two civilians were wounded in another round of shelling in a residential area nearby, police said.

A curfew was imposed on Nasiriyah on Monday, and remained in effect a day later.

Iranian-made rockets were seized in raids in central Nasiriyah, police said.

A day earlier, the U.S. military said at least 20 people were killed by coalition airstrikes in Amarah, another southern city known as a Mahdi Army stronghold. Iraqi officials said 36 people died in fighting that erupted as British and Iraqi forces conducted house-to-house searches.

The U.S. said the fighting involved members of a terror network that imports deadly armor-piercing weapons made in Iran known as "explosively formed penetrators," or EFPs. They also were suspected of bringing militants from Iraq to Iran for terror training, it added.