BAGHDAD (AP) ― A startling spasm of violence shook more than a dozen Iraqi cities Monday, killing over 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 for Monday’s attacks, nearly all of them struck in the capital and in northern Iraqi 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 meters 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 works to overcome a power struggle that pits Sunni and Kurdish leaders against the Shiite prime minister. The infighting, which escalated the day after the U.S. military withdrew last December, has all but paralyzed the government and deepened sectarian tensions around the country.

Iraqi and U.S. officials insist al-Qaida is incapable of sowing the kind of widespread violence that would return Iraq to sectarian warfare. And indeed, Shiite militias so far have held back from returning fire. But Monday’s attacks prove al-Qaida’s continued ability to thwart security, undermine the government and create chaos in a fragile democracy that experts fear is headed toward a failed state.

Iraqi President Jalal Talabani, a Kurd, accused militants of “spreading panic and fear’’ and urged political parties to resolve their differences and help restore stability.

Many of Monday’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 Shiite 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 Shiite 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.

The death toll of at least 110 was the worst for a single day in Iraq since May 10, 2010, when a string of nationwide attacks killed at least 119 people. The sheer breadth of Monday’s bloodshed harkened back to the bloodiest days of Iraq’s sectarian fighting in 2007, when it was common for more than 100 people to die in a day.

It appeared to be the start of a new al-Qaida campaign in Iraq dubbed “Breaking the Walls,” which was announced late last week by the local insurgency’s leader, Abu Bakr al-Baghdadi.

In a statement issued Saturday on a militant website, al-Baghdadi warned that his Islamic State of Iraq is returning to strongholds that it was driven from by the American military. The Islamic State of Iraq is the formal name for the al-Qaida linked group.

“The majority of the Sunnis in Iraq support al-Qaida and are waiting for its return,’’ al-Baghdadi said.

At its peak, al-Qaida in Iraq brutalized its victims with publicized beheadings, suicide bombings and roadside bombs that targeted the Shiite government, the U.S. military and Iraqi civilians alike. In an attempt to goad Shiite militias to respond, Al-Qaida bombed the revered al-Askari Shiite shrine in Samarra in 2006 ― an attack that launched Iraq’s descent into more than three years of sectarian fighting.

But the Iraqi wing of al-Qaida was shunned by the worldwide terror network’s central leadership, which chided it for killing civilians. The insurgency made a series of other missteps ― imposing overly strict Islamic discipline and alienating tribal leaders ― that undercut its support in Iraq’s Sunni communities and helped lead to the widespread defection of fighters to groups allied with the U.S.

As a result, the flow of funding, arms and fighters slowed to a trickle, and al-Qaida in Iraq has struggled to command much power.

Baghdad political analyst Hadi Jalo said the insurgency now feels emboldened by the success of the Sunni-dominated uprising in neighboring Syria against Damascus’ Alawite rulers. The Alawites are an offshoot of Shiite Islam.“It is leading a sectarian war, and Iraq is part of its war and ideology in this region,’’ Jalo said.

Since late last year, Prime Minister Nouri al-Maliki, a Shiite, has courted Sunni tribal leaders to gain their support. With their help, he’s sought to ease the political crisis that has largely broken down along sectarian and ethnic lines. Earlier this month, al-Maliki offered to reinstate former army officers from Sunni provinces who were forced out after the 2003 U.S. invasion because of suspected ties to Saddam Hussein’s regime.

But the political stonewalling shows no sign of breaking, and many of Iraq’s leaders have left Baghdad during the holy Muslim month of Ramadan, which began late last week.

Antony J. Blinken, national security adviser to Vice President Joe Biden, predicted last week that al-Qaida will fail to lure Iraq back toward war. He said the level of violence in Iraq today is roughly what it was before the invasion.

“Iraq remains, relative to other counties, violent, and the Iraqi people suffer from it,” Blinken said in the July 18 briefing at the U.S. Embassy in Baghdad. “But again, I think it’s very important to put all of this in context. Compared to where Iraq was a few years ago, there’s been a dramatic change for the better.”

Statements like that infuriate some Iraqi leaders who say Washington is helping al-Maliki gloss over Iraq’s dire situation.

“Things are not good. Things are bad,’’ Ayad Allawi, the Shiite leader of the secular but Sunni-dominated Iraqiya political coalition said in a July 16 interview with The Associated Press. “The society is split and we don’t have a real democracy ― we have a mockery.”

Bombings and drive-by shootings were virtually unheard-of in Iraq during Saddam’s regime, which kept a tight grasp on society through intimidation and threats. But hundreds of thousands of Kurds and Shiites were either executed or “disappeared’’ during Saddam’s 24-year rule, targeted because of their political opposition.

Sunnis and Kurds complain they have been either sidelined from real authority in the Shiite-led government or blocked by Baghdad from making lucrative regional business deals. Last month, the anti-American cleric Muqtada al-Sadr became the most influential Shiite to join the Sunni-Kurd demand for al-Maliki to resign.

Recent backroom dealing has quieted the recent bickering, and little progress is expected to be made during Ramadan.

However, Monday’s attacks made clear that al-Qaida’s plans to continue its operations in what the Interior Ministry called “a flagrant violation” of “the sanctity of the holy month of Ramadan.”

It was a chilling cause for celebration among jihadists, who quickly went to militant websites and called the wave of violence proof of al-Baghdadi’s new campaign.