A car bomb exploded Saturday near Baghdad's most important Shiite shrine, killing seven people and wounding dozens as authorities imposed new security restrictions to prevent attacks on Shiite pilgrims ahead of major religious ceremonies south of the capital.

The blast occurred around noon in busy Oruba Square, a major commercial area in the Kazimiyah district of Baghdad about 500 yards from the twin-domed shrine of Imam Musa Kadhim, an 8th century Shiite religious leader who is buried there.

An official at the neighborhood hospital said seven people were killed and 30 wounded, including two children. He spoke on condition of anonymity because he was not supposed to release the information.

No group claimed responsibility, but suspicion fell on Sunni religious extremists who consider Shiites as heretics and collaborators with the Americans. Shiites dominate the U.S.-backed Iraqi government.

The blast gouged a small hole in the street, shattered windows in nearby shops and restaurants and set three cars ablaze.

"It was a big explosion," said shop owner Hussein Abdul-Rahman, who suffered minor shrapnel wounds. "I rushed to the scene and saw some dead and injured. Then I felt blood oozing from my back. Then rescuers took me to the hospital for treatment."

Despite the force of the blast, casualties were relatively low because many people were indoors during the broiling noontime heat or had left for the Shiite holy city of Karbala, 50 miles to the south, to attend ceremonies marking the birth of Mohammed al-Mahdi, the 12th and last Shiite imam who disappeared in the 9th century.

Religious Shiites refer to al-Mahdi as the "Hidden Imam," believing he was spared death and will return to Earth to bring peace and justice.

More than one million pilgrims from throughout the Shiite world are expected to converge on Karbala for the celebrations, which reach their high point late Tuesday and early Wednesday.

In the past, Sunni religious extremists, including Al-Qaeda in Iraq, have launched massive and deadly attacks against pilgrims during Shiite celebrations, which have drawn huge crowds since the 2003 ouster of Saddam Hussein's Sunni-dominated regime.

Last March, about 340 people were killed in a weeklong wave of bombings and shootings. Most of the dead were Shiite pilgrims en route to religious ceremonies in Karbala.

To prevent a repeat, Iraqi authorities Saturday banned motorcycles, bicycles and horse-drawn carriages from the streets of Baghdad indefinitely. Earlier in the day, state television announced that the ban applied to all vehicles, including cars and trucks.

Later, the chief military spokesman for Baghdad, Brig. Gen. Qassim al-Mousawi, said cars and trucks would be allowed but other forms of transport that could slip into smaller places were banned until further notice.

All vehicles were ordered off the streets of Baghdad from Aug. 8 through the morning of Aug. 11 during Shiite ceremonies honoring Imam Kadhim, whose tomb was near the site of Saturday's car bombing. The move paralyzed the city but brought killings to a near standstill.

During an interview with government television, al-Mousawi also said U.S. and other international troops would provide water to pilgrims along the route to Karbala. U.S. aircraft would provide surveillance to prevent extremists from firing on the crowds.

He also said pilgrims would be forbidden to carry mobile phones, which can be used to detonate bombs, and large bags which could conceal weapons.

"These measures will provide the right basis for the success of the security plan," al-Mousawi said. "We have taken into consideration all possible threats."

In Karbala, police asked hotel owners not to accept guests who have no passports or residency papers as protection against Sunni extremists infiltrating the crowds. Police said vehicles would be banned from the city center to prevent car bombings near the two mosques that are the focal point of the commemorations.

Rings of police checkpoints will be established around the city to screen pilgrims arriving on foot, said provincial Police Chief Raid Shakir Jawdat.

The U.S. military, meanwhile, announced the grisly discovery of an execution site in the Arab Jabour district, a Sunni Arab area just south of Baghdad where Al-Qaeda in Iraq is known to operate.

During a 24-hour operation on Tuesday and Wednesday, soldiers found human skulls, decomposing bodies in a pit and bones wrapped in bloody clothes, U.S. spokesman Lt. Col. Christopher Garver said in a statement. Troops found blood spatter in a nearby building and other signs that executions had taken place there.

The troops took fire as they entered the area and shot back, killing one suspected insurgent, Garver said. Troops discovered homemade bombs and a weapons cache including trigger wires, he added. Eight suspects were taken into custody.

Elsewhere, U.S. forces killed three insurgent suspects and captured 17 others in a series of raids against Al-Qaeda operatives, the military said.

Among those seized in an operation northeast of Samarra was a man who is suspected of providing money for weapons and other support to foreign fighters, and a suspected "associate" of senior Al-Qaeda leaders who had been sent to the northern industrial city of Beiji to set up operations there, the military said.

Also Saturday, the U.S. command disputed local reports that al-Qaida fighters had launched a series of complex attacks two days before against Iraqi police in Samarra.

"The reports about the Al-Qaeda attack on Samarra are false and we have checked with Iraqis to confirm this never happened," said Lt. Col. Viet Luong, commander of 2nd Battalion, 505th Parachute Infantry Regiment.

The report came from the Iraqi joint coordination center in the provincial capital of Tikrit, which said three people were killed in the attack and nine wounded.