Baghdad ? A bomb placed beneath a parked car exploded outside a prominent Shiite shrine in Baghdad on Saturday, killing at least five people at the start of a major religious ceremony, U.S. military officials said.

Thousands of Shiite pilgrims from across Iraq will soon set off on foot for Karbala, about 60 miles southwest of the capital, to commemorate the birthday of a 9th-century imam, prompting the government of Iraq to impose a partial curfew on some forms of transportation in Baghdad.

Insurgents have frequently targeted such mass congregations of Shiite worshippers. The deadliest day of the war, in August 2005, occurred when a Shiite procession over a bridge in Baghdad turned chaotic after a rumor spread that a suicide bomber was in the crowd. About 1,000 people on their way to the Kadhimiyah shrine died in the resulting stampede.

The bomb Saturday exploded at noon outside the same shrine, the holiest in Baghdad for Shiites, amid groups of day laborers. Iraqi police officials put the death toll as high as nine. At least 27 people were wounded, and several cars and market stalls were destroyed, the U.S. military said in a statement.

The blast came as the first Shiite pilgrims began leaving Baghdad to walk to Karbala for the birthday Tuesday of the “hidden imam,” Muhammad al-Mahdi, who Shiites believe disappeared in the 9th century and whose return will herald an era of peace and justice.

To protect the pilgrims, the Iraqi government imposed a ban in Baghdad on animal-drawn carts, bicycles and motorcycles, which could be used to stage attacks among pedestrians. Iraqi state television had initially announced that the curfew included a ban on all vehicles.

Such curfews “are not very healthy,” said Hashim Hachim, a Sunni member of parliament. “One way for people to feel that life is going on normally is they can go to work and go shopping. … This is going to keep making people feel that there’s not much progress when it comes to security.”

Meanwhile, U.S. troops said they had discovered what they called a “terrorist execution site” in the Arab Jubour area, south of Baghdad. Soldiers found “human skulls, decomposing bodies and bones wrapped in bloody clothes,” the U.S. military said in a statement.