BAGHDAD -- A startling spasm of violence shook more than a dozen Iraqi cities yesterday, killing more than 100 people in coordinated bombings and shootings and wounding twice as many in the country's deadliest day in more than two years.

The attacks came only days after al-Qaida announced it would attempt a comeback with a new offensive against Iraq's weakened government.

With the U.S. military gone and the government mired in infighting, the Iraqi wing of al-Qaida has vowed to retake areas it once controlled and push the nation back toward civil war. Though there was no immediate claim of responsibility, nearly all of yesterday's attacks struck in the capital and in northern cities where al-Qaida can most easily regain a foothold.

"Terrorists are opening another gate of hell for us," said Kamiran Karim, a sweets-seller in the northern city of Kirkuk, which was hit by five exploding cars throughout the morning. He suffered shrapnel wounds when one of the car bombs blew up about 200 yards from his cart.

So far this summer, militants linked to al-Qaida have claimed responsibility for a steady drumbeat of attacks designed to keep the government off balance as it struggles to overcome a power struggle that pits Sunni and Kurdish leaders against the Shia prime minister. The infighting, which escalated the day after the U.S. military withdrew in December, has all but paralyzed the government and deepened sectarian tensions.

Many of yesterday's attacks were stunning in their scope and boldness. They bore the hallmarks of al-Qaida, happening within a few hours of each other and striking mainly at security forces, government officials and Shia neighborhoods.

In one brazen assault, three carloads of gunmen pulled up at an Iraqi army base near the northeastern town of Udaim and opened fire, killing 13 soldiers before escaping, two senior police officials said. In another, a car bomb exploded outside a government office in Sadr City, the poor, sprawling ShiiteShia neighborhood in northeast Baghdad. Sixteen people died.

"The only thing I remember was the smoke and fire, which was everywhere," said Mohammed Munim, an employee at the office who woke up in a nearby emergency room with shrapnel in his neck and back.

The deadliest attack, however, took place just north of Baghdad in the town of Taji, where a double bombing killed at least 41 people. The blasts were timed to hit as police rushed to help victims from a series of five explosions minutes earlier.

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