The woman blew herself up among a group of pilgrims resting on the side of the road in Iskandariyah as women were cooking dinner, men were praying and children were playing nearby, a witness said.
"Minutes after I passed the resting spot, I heard a big explosion. I turned my head back and saw big flames," said Ahmed al-Saadi, a 34-year-old carpenter from Baghdad's Sadr City district.
"We rushed to the site and saw charred bodies, while wounded people were crying for help. Pots and burnt prayer rugs were scattered all over the place," he said.
The Shabaniyah festival, which climaxes over the weekend, marks the birth of Mohammed al-Mahdi, the 12th Shiite imam, who disappeared in the ninth century. Devout Shiites believe he will return to Earth to restore peace and harmony.
At least 26 people were killed and 75 wounded in the attack, a senior police official said, speaking on condition of anonymity because he wasn't authorized to release the information.
He said police believed the bomber was a woman based on witness reports.
Iskandariyah, 30 miles south of the capital, was one of the main cities in an area dubbed the triangle of death for much of the war. But it has seen a recent decline in violence that the U.S. military has attributed to a Sunni movement against al Qaeda in Iraq, as well as an influx of American troops.
Sporadic attacks continue to hit the area, however.
On Feb. 24, another suicide bomber killed at least 40 Shiite pilgrims near Iskandariyah as they were en route for another religious festival.
Shiite pilgrims also faced violence in Baghdad on Thursday.
A roadside bomb exploded in the southeastern district of Zafaraniyah, killing one Iraqi and wounding as many as nine others, including six pilgrims, officials said.
Lt. Col. Steve Stover, a spokesman for U.S. forces in Baghdad, blamed a "special groups cell in the area" for the attack, saying it was seeking to heighten public unrest.
The U.S. military uses the term special groups to refer to Shiite extremists it says are backed by Iran and refusing to adhere to a cease-fire by anti-U.S. cleric Muqtada al-Sadr.
Police said another roadside bomb killed one pilgrim and wounded seven others - all young men - in the central Alwiya district. But Stover said that bomb was targeting a U.S. patrol and caused no injuries.
Authorities have fortified the capital and Karbala, 50 miles to the south, to prevent extremists from infiltrating the main areas where the rituals are performed. But pilgrims remain vulnerable as they walk on foot to the shrines.
Shiite religious festivals have often been targeted by militants from al Qaeda in Iraq, the country's deadliest Sunni terror group.
The most recent attack against Shiite pilgrims was last month, when three female suicide bombers struck Shiite pilgrims in nearly simultaneous bombings in Baghdad, killing at least 32 people and wounding more than 100.
On Thursday, Brig. Gen. Qassim al-Moussawi, the chief military spokesman for Baghdad, issued several regulations designed to defuse sectarian tensions during the journey to Karbala. The route passes through Sunni areas just south of the capital.
The order banned members of the Shiite-dominated security forces deployed along the route from plastering their vehicles with religious and political symbols - like images of Shiite saints or Shiite party posters - and from joining pilgrims in their religious chants. Security personnel, al-Moussawi said, must stick to the national anthem and fly the national flag from their vehicles.
Al-Moussawi, who announced the regulations on state television, also said pilgrims must not carry arms and those traveling on foot mustn't walk about after dark. He warned against rumors that could sow panic and against accepting food from strangers.
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