A wave of bombings across Iraq killed dozens of people Thursday morning, security officials said, in a grim indication of the strength of the insurgency two months after the U.S. military completed its withdrawal.

Most of the attacks, which were carried out with car bombs and small arms, appeared to target security forces in the capital and other cities, authorities said. At least 55 people were killed and more than 220 wounded, according to local security officials, most of whom spoke on the condition of anonymity.

Although Thursday was not the deadliest day in Iraq since American forces completed their departure in late December, the attacks represented the most widespread operation yet mounted by suspected Sunni insurgents who have sought for years to topple the Shiite-led government in Baghdad.

Several civilians were among the victims, including some schoolchildren, security officials said.

The Islamic State of Iraq, the Iraqi branch of al-Qaeda, claimed responsibility for the attacks late Thursday. The group said it targeted security forces and government officials in “revenge for the elimination and torture campaigns that Sunni men and women face in the prisons of Baghdad and other cities,” the Associated Press reported early Friday.

Iraqi officials did not provide an official death toll, and few appeared on television to speak about, or condemn, the attacks. Osama al-Nujaifi, the speaker of parliament, issued a statement saying that the attacks represented an attempt to “flare up strife” among Iraqis.

Nujaifi said the assailants might have been trying to ignite fears about security to dissuade regional officials from attending the annual Arab summit scheduled to be held in Baghdad next month.

Lawmakers on Thursday approved the purchase of 350 armored vehicles, worth more than $50 million, for their personal use.

As the casualties mounted Thursday, Iraqis reacted with outrage and blamed the country’s fractured political leadership for the insecurity that continues to plague the country.

“Today’s events mean that we have no government or that we have a weak one,” said Waleed al-Rubaie, 34, a private-sector worker. “The political disputes are behind today’s blasts.”

Tension among Iraqi politicians has soared since U.S. troops left, most notably after the issuance of an arrest warrant for the country’s Sunni vice president, Tariq al-Hashimi. Prime Minister Nouri al-Maliki, a Shiite, accused him of being involved in terrorist acts, a charge Hashimi denied. The vice president has avoided arrest by staying in the semiautonomous Kurdish region in the north.

Wesam al-Auqali, 35, a blacksmith, said security forces are unprepared and susceptible to bribes. Auqali said he recently drove a truck loaded with construction materials into a neighborhood where such vehicles are banned because of the threat of car bombs. All it took to get inside was a $4 payoff to a police officer, he said.

“You can imagine how easy it is to get a car bomb past,” he said. “They can get as many as they want through a checkpoint.”

Thursday’s carnage followed a relatively quiet period in Baghdad and other often-violent cities, a lull that had led some Iraqis to speculate that Sunni insurgents had flooded into neighboring ­Syria to join the revolt against President Bashar al-Assad.

Baghdad bore the brunt of the latest attacks, with at least 23 dead, but assailants also struck in the northern provinces of ­Salahuddin and Kirkuk, in Anbar province in the west and in Babil province, south of Baghdad.

In the Adhamiyah district of northern Baghdad, assailants raised the black flag of the Islamic State of Iraq.

Londoño reported from Irbil, Iraq. Special correspondent Aziz Alwan in Irbil contributed to this report.