A suicide bomber driving a car killed at least 20 Shi'ite Muslim pilgrims at a bus station in the Iraqi town of Mussayab on Thursday, during the peak of a Shi'ite religious rite, police and medics said.
The attack underlines sectarian tensions that threaten to further destabilise the country a year after U.S. troops left.
Police said the bomber drove into a busy station where pilgrims were gathering to return to Baghdad and the northern provinces on their way back from the holy city of Kerbala, where thousands of Shi'ites make an annual pilgrimage for the Arbain rite.
Mussayab is located some 60 km (40 miles) south of the capital Baghdad.
"I was getting a sandwich when a very strong explosion rocked the place and the blast threw me away. When I regained my senses and stood up, I saw dozens of bodies, said Ali Sabbar, a pilgrim who witnessed the explosion.
"Many cars were set on fire. I just left the place and didn't even participate in the evacuation of the victims".
Arbain has been a frequent target for militants since the U.S.-led invasion that toppled Sunni dictator Saddam Hussein, who banned Shi'ite festivals.
The latest violence followed nearly two week of protests against Shi'ite Prime Minister Nuri al-Maliki by thousands of people from the minority Sunni community in the western province of Anbar.
Although violence is far lower than during the sectarian slaughter of 2006-2007, a total of 4,471 civilians died last year in what one rights group described as a "low-level war" with insurgents.