Updated at 12:38 p.m. Eastern.
Twin car bombs tore through a crowd of Shiite pilgrims packing a highway as they walked to a holy city south of Baghdad on Friday for a major religious observance, killing at least 40 people and wounding 154 others, Iraqi officials said.
It was the third deadly bombing this week hitting the ceremony in which hundreds of thousands of Shiites have been converging on the city of Karbala. Friday's attack struck during the culmination of the pilgrimage.
Friday's attack began shortly after noon when a parked car bomb exploded just east of one of three main entrances to Karbala, two Health Ministry officials said. The explosion sent throngs of pilgrims running down the highway and straight into the path of a suicide car bomber who detonated the vehicle, they said.
At least 154 people were wounded in the consecutive blasts, the officials said.
However, an Iraqi police official reported it was two mortar rounds that struck the area, followed by a suicide car bomb. Such conflicting accounts are common in the chaotic aftermath of bombings in Iraq.
Witnesses described widespread panic as people tried to carry the dead and wounded through a thick crowd of pilgrims packed shoulder to shoulder. Some were injured as people tried to run from the blast site, said Kareem Madhi, a pilgrim from nearby Hillah.
"I saw a fireball and then black smoke raising," he said. "The security measures are unable to protect these huge numbers of pilgrims."
Iraqi police tried to prevent journalists from reaching the scene.
At least 108 people were wounded in the attack, the official said, adding that the area was clogged with pilgrims and the number of casualties could increase. A hospital official in Karbala, also speaking on condition of anonymity, confirmed the number of dead and wounded.
Hours earlier, two separate roadside bombs targeting Shiite pilgrims exploded in Baghdad, killing one and wounding seven others, a security official in the capital said.
The official said the first attack took place Wednesday at about 6:15 a.m. in western Baghdad, killing one person and wounding three. He said a second explosion in southwest Baghdad wounded four pilgrims.
The official spoke on condition of anonymity because he was not authorized to release the information.
For years, the main Shiite pilgrimages have been a prime target for attacks blamed on Sunni extremists seeking to widen sectarian rifts. The current processions have brought hundreds of thousands of people streaming toward Karbala to end the 40-day mourning period marking the death of Hussein, a revered Shiite figure from the 7th century.
The religious events also come about a month before critical parliamentary elections on March 7, which are seen as a test of reconciliation between majority Shiites and minority Sunnis who dominated Iraq until Saddam Hussein's ouster in 2003.
On Monday, a woman suicide bomber north of Baghdad, including a dozen children. After the attack, authorities promised to intensify security along the pilgrimage route.
Karbala police spokesman, Maj. Alaa Abbas, said more than 30,000 security personnel were deployed around the city. The measures included bomb-sniffing dogs at the three main entrances to Karbala and undercover intelligence agents in the crowds.
But tight security in the past has failed to prevent bloodshed among Shiite pilgrims or at shrines.
During a Shiite pilgrimage in February 2009, a female suicide bomber attacked a tent filled with women and children resting during the walk to Karbala, killing 40 people and wounding 60 others. A month before that, a suicide bomber dressed in women's clothing and hiding among Iranian pilgrims killed more than three dozen people outside a mosque in Baghdad's Shiite neighborhood of Kazimiyah.
Earlier Wednesday, a senior security official said agents arrested 13 suspects believed involved in making explosive belts for suicide attacks.
The official said authorities believe the belts - including five seized by police - were intended for attacks on Shiite pilgrims. The official spoke on condition of anonymity because he was not allowed to give the information to media.