At least 72 people were killed Thursday in a series of attacks on Shiites in Iraq, marking the deadliest day since U.S. troops withdrew last month and raising new worries about the country’s sharp sectarian divisions.

Analysts warned that the bombings, in what appeared to be the worst day of violence in the country in more than a year, could unleash more danger. One local security official blamed the insurgent group al-Qaeda in Iraq, but no militant organization asserted responsibility for the blasts.

The attacks, which wounded at least 147 people, included explosions in two Shiite neighborhoods in the capital and a suicide bombing next to a large group of Shiites on a pilgrimage in southern Iraq. The violence occurred against the backdrop of a deepening political standoff between the country’s Shiite and Sunni leaders.

A majority of the Sunni-supported Iraqiya political bloc boycotted parliamentary proceedings Thursday. Members of the group accuse Prime Minister Nouri al-Maliki, a Shiite, of trying to dissolve a political framework established under U.S. guidance to ensure power-sharing among Shiites, Sunnis and Kurds.

“Rather than sharing power, he wishes to create turbulence between political parties of Iraq for the purpose of maintaining and consolidating his own power,” Haider al-Mulla, a spokesman for Iraqiya, said this week.

In an e-mailed statement, Ali Hadi al-Moussawi, a spokesman for Maliki, said terrorists are trying to ignite a war between Shiites and Sunnis. “That is why you see them attack such religious symbols like the Shiite practice of pilgrimage,” Moussawi said. He also said Iraqiya leaders are aggravating sectarian tensions.

Just a day after the U.S. military’s Dec. 18 departure, Maliki’s security forces accused Tariq al-Hashimi, the Sunni vice president, of running a hit squad, prompting him to flee the capital and igniting the political crisis.

In Washington, State Department spokeswoman Victoria Nuland condemned the latest “acts of terror.” “They are desperate attempts by the same kind of folk who’ve been active in Iraq trying to turn back the clock,” she said. Nuland said Vice President Biden and other administration officials are continuing to press Iraqi leaders to hold formal talks aimed at ending the political conflict.

“We are quite encouraged that a number of Iraqi politicians are also calling for such a meeting, which we hope takes place soon.”

Lt. Col. Joel Rayburn, an Iraq analyst at the National Defense University who earlier served as a U.S. intelligence officer in the country, said the attack on the Shiite pilgrims could embarrass Maliki’s government, which has staked its reputation on protecting such pilgrimages. If the attacks continue, they could spark a reemergence of the Shiite Mahdi Army, a militia that was created by firebrand cleric Moqtada al-Sadr to protect Shiites when the government appeared unable to do so.

“And there won’t be anything Maliki can say or do against it,” said Rayburn, stressing that he was expressing his opinions and not those of the university.

Series of attacks

The suicide bombing about 200 miles southeast of the capital, near Nasiriyah, targeted Shiites who were on the road to the holy city of Karbala to honor a venerated Shiite martyr, Imam Hussein. At least 48 people were killed and 81 were wounded in the attack, a provincial security chief said.

Just before the bombing, an Iraqi army officer spotted the assailant and tried to intervene, said the security chief, Sajad al-Asadi. The officer attempted to wrap his arms around the bomber and tackle him before he could detonate his explosives. The officer was killed in the bombing, Asadi said.

The security chief said that many of the injured are in serious or critical condition and that the death toll is likely to rise. He blamed al-Qaeda in Iraq for the attacks.

“This is al-Qaeda’s tactic to target Shiite pilgrims.”

On Dec. 22, more than a dozen bombs were set off within two hours, killing at least 65 people. The Islamic State of Iraq, a group affiliated with al-Qaeda in Iraq, later asserted responsibility.

The Thursday bombing of the pilgrims followed blasts earlier in the day in two Shiite neighborhoods in Baghdad.

An explosives-laden motorcycle blew up near a group of day laborers in Sadr City, a sprawling Shiite slum named for Sadr’s father, a senior cleric who was assassinated during the rule of Saddam Hussein.

Two more bombs were detonated simultaneously near a hospital in the same area, said Interior Ministry officials, who discussed the attacks on the condition of anonymity because they were not authorized to speak publicly.

Nine people were killed and 35 were injured in the blasts, the officials said.

Ninety minutes later, two car bombs exploded near Aruba Square in the Kadhimiyah district of northern Baghdad, killing 15 and injuring 31, according to initial reports.

Prospect of danger, hope

Kenneth Pollack, director of the Saban Center for Middle East Policy at the Brookings Institution, said it is impossible to know for certain at this point who is responsible for the attacks. It could be an al-Qaeda group or other terrorists seeking to prove that the government is powerless to stop them, he said, or Sunni insurgents who do not like Maliki’s quest for power.

The real danger, Pollack said, is how armed Shiite groups interpret the bombings. If they see them as evidence of Sunnis trying to hurt Maliki’s government, they might attack Sunni civilians. “That becomes a very dangerous situation,” he said.

But Reidar Visser, an Iraq expert at the Norwegian Institute of International Affairs, noted one positive sign: There was “immediate and robust” condemnation of the attacks across Iraq’s political spectrum, including from prominent Sunnis.

Maliki’s spokesman said the attacks did not expose major failures in security because the pilgrims were out in the open, traveling by the thousands. But the attacks may indicate a need for better intelligence gathering, Moussawi said.

Others, however, said the country’s leadership was at fault.

“They’re busy doing the wrong things,” said parliament member Mahmoud Othman, an independent Kurd who said he was including all parties and sects in his critique. “They’re busy with conflict, day and night, and don’t have time for security.”

Terrorists are taking advantage of the vacuum, Othman said, and are willing to attack both Shiite and Sunni civilians to incite militias sympathetic to either side. “They want to create an armed, sectarian conflict.”

Staff writer Joby Warrick in Washington and special correspondents Aziz Alwan and Asaad Majeed in Baghdad contributed to this report.