Thousands of pilgrims arrived on foot Saturday at a Shiite shrine in Baghdad to start a major religious commemoration, as private vehicles were banned from the streets to prevent car bombings by Sunni extremists. At least 18 people including a U.S. soldier were killed in the political and sectarian violence.

In a statement, Prime Minister Nouri al-Maliki urged Iraqis to cooperate with security forces during the ceremonies marking the death in 799 A.D. of Imam Moussa Kadhim, one of the 12 Shiite saints.

The imam is buried in a golden-domed shrine in north Baghdad's Kazimiyah district. Soldiers, police and Shiite volunteers threw a security cordon around the shrine, frisking pilgrims as they arrived.

CountryWatch: Iraq

Tens of thousands more Shiites are expected to visit the shrine on Sunday, when the ceremonies peak. Fearing an attack among the pilgrims, the government banned all private vehicles on the streets from Friday night until Monday morning, and announced a public holiday on Sunday, which is normally a working day in the Islamic world.

Mindful of Sunni-Shiite tensions, al-Maliki, a Shiite, warned against turning the ceremonies into a political demonstration, calling on clerics to urge people to unite and "shun whatever could lead to sectarian fights."

"We warn all those who use podiums (in mosques) to incite sectarian violence that they will be prosecuted as terrorists," he said without elaboration.

Shiites from throughout the country began arriving at the shrine on Friday night on foot. Late Friday, unidentified gunmen opened fire on a group of pilgrims walking through the mostly Sunni Adil neighborhood in western Baghdad, killing seven of them.

Three mortars landed in Kazimiyah district late Saturday — two in a river and one on a school compound — but caused no casualties.

Last year, the government said about 1,000 people died during the Imam Kadhim commemoration when rumors of suicide bombers triggered a mass stampede on a bridge across the Tigris River. It was the biggest single day death toll since the U.S.-led invasion in March 2003.

"Last year I was on the bridge and I fell into the water but that gave me the power to come back. I challenge the terrorists now that I have come to visit the imam," said Rahim al-Rubaie, 29.

Shiites were prevented from mustering huge crowds at religious ceremonies during Saddam Hussein's Sunni-dominated regime. But since Saddam's ouster in 2003, Shiite politicians and religious leaders have encouraged huge turnouts as a demonstration of the majority sect's power.

As the pilgrims arrived under the blistering summer heat, volunteers handed out orange drinks and free food. Many of the pilgrims waved the green flag of Islam or flags of their tribes, and some were cloaked in white shrouds, a symbol of their willingness to die.

"My cousins died here last year so I have come this year and I wish to die to be martyr and join my cousins in the paradise " said Ali al-Saedi, a 20-year-old college student.

The ceremonies are taking place during a major U.S.-Iraqi security operation aimed at curbing Sunni-Shiite violence, which threatens the stability of the new government of national unity. Nearly 12,000 U.S. and Iraqi troop reinforcements are coming in to take control of this city of 6.5 million people neighborhood by neighborhood.

A U.S. soldier assigned to 1st Brigade, 1st Armored Division died Saturday in Anbar province, the stronghold of the Sunni Arab insurgency, a U.S. military statement said.

Nine Iraqis were killed Saturday in Baqouba, a major Sunni-Shiite flashpoint, 35 miles northeast of Baghdad. The victims included two professors of the Diyala University who were shot dead while returning home.

Also Saturday, four Iraqi soldiers were killed when the convoy they were traveling in was struck by a roadside bomb in Diwaniyah, 80 miles of Baghdad, police 1st Lt. Raed Jabir said.

In addition, five more people were killed Saturday in scattered violence across the country.

Meanwhile, Pope Benedict XVI has appealed for the release of an Iraqi priest, Rev. Hanna Saad Sirop, kidnapped at gunpoint in Baghdad, in a telegram sent to the Patriarch of the Iraqi capital, Italian news agencies reported Saturday.