Baghdad, Iraq ? Insurgents struck Baghdad with at least a dozen attacks that targeted Shiite Muslim civilians, Iraqi security forces and American troops, killing more than 160 people in the deadliest day of violence in the capital since the U.S. invasion.

U.S. military officials said the daylong waves of suicide bombings, rocket attacks and shootings across the city bore the hallmarks of al-Qaida in Iraq, the radical Sunni Muslim insurgent organization led by Abu Musab al-Zarqawi.

The group did not immediately assert direct responsibility for the attacks, but an Internet statement issued in its name welcomed the start of “the revenge battles throughout the land of Mesopotamia.”

The statement linked the attacks to a U.S. and Iraqi offensive underway against insurgents in the northern city of Tal Afar, and a subsequent audio recording attributed to Zarqawi that was posted on the Internet accused the Shiite-led Iraqi government of having declared war on the Sunnis of Tal Afar. As a result, al-Qaida in Iraq “has decided the launch a comprehensive war on the Shiites all over Iraq, wherever and whenever they are found. This is revenge. … Take care, because we are not going to have mercy on you,” the recording said, according to a translation by the Washington-based SITE Institute, a group that monitors radical Web sites.

The attacks appeared calculated to undermine public faith in the ability of the fledgling government to protect its people, by showing that insurgents can strike in Baghdad despite the U.S. and Iraqi military efforts to stop them. Some of Wednesday’s attacks were carried out in ways that maximized death tolls.

U.S. helicopters hover as thick smoke rises after an explosion in central Baghdad. Several large blasts shook downtown Baghdad on Wednesday, sending plumes of billowing black smoke over the Iraqi capital. More than 160 people died in the bombings.

In northwest Baghdad, a driver in the heavily Shiite neighborhood of Kadhimiyah pulled up alongside a gathering point for day laborers and offered jobs, witnesses said. He waited until a crowd of workers had clustered around his four-door car, then detonated explosives packed inside, said 20-year-old Salim Hussein, who witnessed the attack.

The blast killed at least 112 people and wounded hundreds of others.

Within an hour of the first attack, another driver smashed his car into two other vehicles at an intersection, then blew up his car in a fireball when a crowd gathered, police said. At least 15 people died, police Lt. Mustafa Majid said.

More attacks were mounted throughout the day, signaled by rattling booms, black smoke and U.S. military helicopters shuttling across the sky. Traffic on main roads shut down as police closed key routes. Rumors spread that more car bombers were roaming the city and that men wearing suicide belts were infiltrating hospitals.

The other attacks included two car bombings that killed a total of 26 people, one that targeted an Iraqi army convoy and killed three soldiers, and two that hit U.S. military convoys, U.S. and Iraqi officials said. A U.S. military spokesman, Lt. Col. Steve Boylan, said he knew of three American troops wounded in the day’s attacks and that none had been reported killed.

Separately, attackers opened fire on a car carrying Iraqi police officers, killing one, and then detonated a car bomb when other officers responded, killing four more people, police said. A separate rocket attack killed two Iraqi civilians.