At least 51 people died when two car bombs blasted through security checkpoints ringing the Iraqi holy city of Karbala yesterday.
Most of the victims were Shiite pilgrims travelling 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 US 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.
They 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 injured in the near-simultaneous blasts set off by suicide bombers driving cars packed with explosives.
Ali Khamas, a pilgrim from 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, 42, a lorry driver. "After the explosion, people started to run in all directions, while wounded people on the ground were screaming for help."
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 series of attacks this week shattered a relative calm since the formation last month of a new government under Prime Minister Nouri al-Maliki, a Shiite whose family comes from the Karbala area.
The bombings were the latest in a three-day barrage of attacks across Iraq that have killed more than 120 people since Tuesday.
Earlier yesterday, 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, strike on Shiite pilgrims walking to Karbala from Baghdad killed one and wounded 10 of them.