© Xinhua/Reuters PhotoImage from an August 3, 2010 bomb attack in Kut, Baghdad
A pair of car bombs blasted through security checkpoints ringing the Iraqi holy city of Karbala Thursday and killed at least 51 people, most of whom were Shiite pilgrims headed to observe yearly religious rituals.

It was the latest in a wave of attacks in recent days, as insurgents test Iraqi security forces ahead of the planned U.S. withdrawal at the end of the year.

There was no immediate claim of responsibility, but the bombings bore the hallmark of al-Qaida and other Sunni-dominated extremist groups that frequently target Shiite pilgrimages in hopes of re-igniting sectarian violence that brought Iraq to the brink of civil war just a few years ago.

Authorities estimated as many as 183 people were wounded in the near-simultaneous blasts set off by suicide bombers driving cars packed with explosives.

Ali Khamas, a pilgrim from the Shiite neighborhood of Sadr City in Baghdad, said he saw a car speeding toward one of the checkpoints, its driver refusing to stop despite warnings screamed by Iraqi soldiers.

"He sped up and blew up his car near the checkpoint," said Khamas, a 42-year-old truck driver. "After the explosion, people started to run in all directions, while wounded people on the ground were screaming for help. I saw several dead bodies on the ground."

Still, Khamas said, the pilgrims continued to head to Karbala: "It will not deter us from continuing our march to the holy shrine ... even if the explosions increase."

Mohammed al-Moussawi, head of the Kerbala provincial council, gave a different account, saying two cars parked outside the checkpoints to the city had exploded at the same time.

Pilgrims flock to donate blood
Hundreds of thousands of Shiite pilgrims are pouring into Karbala for Arbain, which marks the end of a 40-day mourning period for Imam Hussein, a grandson of the Prophet Muhammad. Killed in a 7th century battle, Imam Hussein is a central figure of Shiite Islam.

The festival culminates on Tuesday.

About 120,000 Iraqi police and troops have been mobilized to protect the pilgrims, many of whom walk for days to Karbala.

Crowds of pilgrims headed to a Karbala hospital to donate blood for the wounded. Authorities said 11 soldiers and policemen were among the dead, the rest were pilgrims.

Iraqi security officials and hospital staff who gave details on the attacks and casualty figures all spoke on condition of anonymity because they were not authorized to release information.

Adil Barwari, an adviser to Prime Minister Nouri al-Maliki, a Shiite whose family comes from the Karbala area, said the attacks show extremists' determination "to undermine the new Iraqi government."

He noted that insurgents also likely hope to spook other nations who are sending delegates to the Arab League summit in Baghdad in March - the first time Iraq has hosted the meeting in 20 years.

"They want it to fail," Barwari said. "But al-Qaida will not succeed in achieving this goal."

Attacks against Shiite pilgrims have been a tactic of Sunni extremists since the early years of the Iraq conflict.

Shiite politicians encouraged huge turnouts at religious celebrations to dramatize the power of the Shiite majority after the fall of the Sunni-dominated regime of Saddam Hussein.

"The enemies always develop their tactics and improvise new plans to make use of any security breach," said Karbala provincial councilman Shadhan al-Aboudi.

He blamed the blasts on al-Qaida and Saddam loyalists: "They have apparently found a gap today in the security measures and they carried out an evil act against innocent believers who were practicing religious rituals."

The bombings were the latest in a three-day barrage of attacks across Iraq that have killed more than 120 people since Tuesday.

Earlier Thursday, a suicide bomber rammed his explosives-packed car into the front gate of a police headquarters in the eastern Iraqi city of Baqouba, killing three. Another, earlier strike on Shiite pilgrims walking to Karbala from Baghdad killed one and wounded 10 of them.

A day earlier, on Wednesday, another suicide bomber killed seven people after he blew up the ambulance he was driving into a security compound run by guards tasked with protecting government facilities in Baqouba.

And on Tuesday, 65 people died when a suicide bomber set off his explosives-packed vest in a crowd of police recruits in Saddam Hussein's northern hometown of Tikrit.

The Associated Press and Reuters contributed to this report.