Despite the violence, hundreds of thousands of worshippers streamed toward the twin golden domed mosques amid tight security in the city south of Baghdad. Men and women filed through separate metal gates to be searched.
The explosives-laden car blew up about 9 a.m. near minibuses assembled to pick up pilgrims in Baghdad's mainly Shiite district of Shaab.
It was the latest in a series of bombings targeting Shiites heading to Karbala for a major religious festival that culminates this weekend. The deadliest was a female suicide bombing on Thursday that killed at least 18 pilgrims resting on the side of the road south of Baghdad.
A passenger van packed with explosives also blew up Friday night at a bus station in Balad, north of Baghdad where Shiite pilgrims had stopped for the night, killing four people and wounding dozens, U.S. and Iraqi officials said.
The attacks have raised concern that extremists were seeking to re-ignite the firestorm of sectarian massacres that plunged Iraq to the brink of civil war two years ago before thousands of American reinforcements were rushed to the country.
U.S.-backed Iraqi troops have stepped up security measures for the pilgrimage, but travelers remain vulnerable on the road.
In Karbala, women in flowing black robes were searched by female guards at a separate checkpoint from the men. Iraqi security forces were taking no chances amid warnings that insurgents are increasingly using women to stage attacks
- women can more easily hide explosives under their abayas and men are reluctant to search them.
AP Television News video showed Iraqi security forces picking through plastic bags as men in T-shirts waited patiently.
Posters of Shiite religious leaders were confiscated for fear they might provoke attacks by followers of rival clerics, and cell phones were banned because they could be used to trigger bombs.