Masked gunmen on Tuesday assassinated a Sunni Arab cleric who headed the city council in once-restive city of Fallujah, and two bombs exploded minutes apart near a central Baghdad square, killing at least seven people and wounding 20.

Also Tuesday, the U.S. military said four Marines died in separate explosions in western Iraq.

In Fallujah, Sheik Kamal Nazal was gunned down in a hail of gunfire from two passing cars as he walked to work, said Fallujah's police chief, Brig. Hudairi al-Janabi.

The motive for the attack was unknown, but it appeared to be part of an insurgent campaign to prevent Sunni Arab leaders from joining the political process.

Last month, Nazal welcomed Prime Minister Ibrahim al-Jaafari and U.S. Ambassador Zalmay Khalilzad to the city 40 miles west of Baghdad. Iraq's Sunni Arab insurgents have previously targeted Iraqis deemed as collaborating with the country's U.S.-backed authorities.

"Sheik Nazal was the first among those who carried weapons to protect government offices after the (U.S.) invasion in 2003," said Dr. Salman al-Jumaili, a senior member of the Iraqi Islamic Party. "We hold the Iraqi government and occupation forces responsible for bringing all this suffering and damage to this city."

Nazal, who was in his 60s, was elected council chairman by Fallujah's residents in early 2005. He was previously arrested along with his brother by U.S. troops before the April 2004 Fallujah offensive.

Fallujah had been the headquarters of Iraqi insurgent and religious extremist groups until U.S. forces overran the city in November 2004. Since then the city has become among the most tightly controlled in Iraq.

In Baghdad, Iraqi security forces detained at least 26 suspected Sunni Arab insurgents planning to attack Shiite pilgrims during Ashoura commemorations, which climax Thursday. The ceremonies mark the 7th century death of revered Shiite saint Imam Hussein.

The first Baghdad bomb was in a plastic bag placed near a CD vendor's stand close to the capital's Tahrir Square by a man who fled seconds before the explosion, which killed at least three people, said police Capt. Mohammed Abdul Ghani. Ten minutes later, a second bomb hidden in a drain exploded, killing four, including one policeman, he said.

Officials at two hospitals where the casualties were taken confirmed that seven people were killed and at least 20 wounded in both bombings.

The exact motive of the first bomb was unclear. Police Lt. Mohammed Khayoun said the CD vendors may have been targeted because some sold sexually explicit films. The second blast, he added, was aimed at policemen arriving at the scene.

A witness, however, said the first blast occurred near a crowd of people watching a film on the martyrdom of Imam Hussein, the Shiite saint.

"I was standing near the vendor who was targeted. He had a television set showing a film on the martyrdom of Imam Hussein and then the explosion happened," said Ali Abdul Mohsen Karim, 25, who sells leather jackets in the square. "I saw two people dead and ran and hid in one of the stores. Then the other explosion took place."

Ashoura commemorations reach their height on Thursday, when hundreds of thousands of Shiites are expected to take part in self-flagellation processions across the country.

Iraqi security forces are on high alert to prevent a repeat of the past two Ashoura ceremonies in which Sunni Arab extremist suicide attackers detonated explosions targeting Shiite worshippers, killing more than 230 people in Baghdad and the holy southern city of Karbala.

About 100 Iraqi security forces, some repelling from U.S. helicopters, raided a suspected insurgent training camp in Salman Pak, 20 miles southeast of Baghdad, late Monday and detained 26 people allegedly preparing to attack Shiite pilgrims.

The U.S. military said the operation targeted insurgents planning attacks on Shiite Muslims bound for Karbala, 50 miles south of Baghdad, for Ashoura services.

The raid and Baghdad bombings came amid a spate of sectarian-related killings carried out by rival armed Shiite and Sunni Arab groups vying for ascendancy in post-Saddam Hussein Iraq.

Several political parties are also trying to form a national unity government comprising members of the country's Shiite, Sunni Arab and Kurdish communities. The United States hopes such a government could help reduce the incessant violence and allow coalition forces to reduce their troop levels.

The four U.S. Marines were killed by bombings in western Iraq's volatile Anbar province in the past few days, the military said Tuesday.

Three Marines assigned to the 22nd Marine Expeditionary Unit died in a bombing Monday in Hit, 85 miles west of Baghdad, according to a statement. The victims had been in Anbar province since mid-December working with an Iraqi army battalion.

The other Marine, attached to the 2nd Marine Division, II Marine Expeditionary Force, died from wounds caused by a bomb blast Sunday in an unspecified location in Anbar, which includes the cities of Fallujah and Ramadi.

The latest deaths bring the number of U.S. military personnel killed to at least 2,257 since the Iraq war began in March 2003, according to an Associated Press count.

The blasts highlight the constant danger posed by homemade bombs across Iraq, which have become the deadliest weapon used by insurgents.