BAGHDAD -- Gunmen assassinated a top aide of anti-American leader Muqtada al-Sadr on Friday, sharpening a Shiite power struggle that has already triggered fighting between the cleric's followers and the U.S.-backed Iraqi government.
Riyadh al-Nouri, director of al-Sadr's office in Najaf, was gunned down by an unknown number assailants near his home after returning from prayer services, police and Sadrist officials said.
Al-Sadr blamed the Americans and their Iraqi allies for the killing but called for calm -- presumably to avoid a showdown at a time his Mahdi Army militia is under pressure by Iraqi and U.S.-led forces in Baghdad and southern Iraq.
Prime Minister Nouri al-Maliki, also a Shiite, condemned "this savage crime" and ordered an investigation "to pursue and arrest the killers." But many of the 5,000 people who attended al-Nouri's funeral later Friday in Najaf chanted "al-Maliki is the enemy of God" as they shouted slogans against al-Sadr's Shiite political rivals.
Authorities declared a curfew in Najaf, the world's premier Shiite theological center 100 miles south of Baghdad. Security forces took to the streets in several major cities across the Shiite south. A curfew was also imposed in Hillah, where government troops clashed with al-Sadr's militia last month.
The assassination of such an influential Sadrist figure is likely to increase tension between al-Sadr's movement and the Shiite-led government.
Several prominent Sadrists described al-Nouri as a voice of moderation within the movement, arguing against an armed confrontation with the Americans and al-Sadr's Shiite rivals. He had also opposed a decision by the Sadrists last year to withdraw from al-Maliki's government.
Al-Nouri, 41, was one of al-Sadr's closest aides. Al-Nouri's sister is married to one of al-Sadr's brothers. As director of the Najaf office, al-Nouri was al-Sadr's representative in the world's most prestigious center of Shiite learning.
Al-Nouri and another top al-Sadr lieutenant, Sheik Mustafa al-Yacoubi, were detained by American soldiers in May 2004 in the killing a year earlier of a moderate Shiite cleric, Sheik Abdul-Majid al-Khoei, in Najaf shortly after the U.S.-led invasion of Iraq.
An arrest warrant was issued for al-Sadr himself but was never served. The warrant and the closing by U.S. authorities of al-Sadr's newspaper triggered massive uprisings that engulfed Shiite areas of central and southern Iraq.
Al-Nouri and al-Yacoubi were freed in 2005 as part of an agreement to end the Sadrist rebellion, which claimed several thousand lives.
Tension between al-Sadr and other Shiite parties exploded into violence last month when al-Maliki launched an ill-planned offensive against Shiite militias and gangs in Basra, Iraq's second largest city.
The offensive faltered after al-Sadr's militia launched attacks throughout the south and in Baghdad, where militants showered the U.S.-controlled Green Zone with rockets and mortars, killing four Americans.
Clashes have continued in Baghdad and Basra, despite al-Sadr's order March 30 for his militiamen to stand down under a deal brokered in Iran.
American and Iraqi officials insist the Basra crackdown was not aimed at al-Sadr's political movement but at criminals and Iranian-backed splinter groups. But Sadrists believed their Shiite rivals in government were trying to weaken their movement before provincial elections this fall.
One of those rival parties, the Supreme Islamic Iraqi Council, controls the security services in Najaf. Sadrist officials said al-Nouri was slain about 300 yards from a security checkpoint but that it took police about 10 minutes to respond to the sounds of gunfire.
In his statement Friday, al-Sadr, who is believed to be in the Iranian holy city of Qom, blamed the killing on "the hands of the occupiers and their stooges reaching out traitorously and aggressively against our dear martyr," a reference to the U.S. and its Iraqi allies.
"I call upon Sadrist followers to be patient," said al-Sadr, who is under enormous pressure from all Iraqi political parties to disband his militia, his most important instrument of power.
"The occupiers will not rest in our land as long as I am alive," he said. "We demand the government open an investigation and punish the criminals. We call upon all political and religious groups to work toward ending the killing of clerics."
President Jalal Talabani, a Kurd, described the killing as an attempt "to destabilize the country" and encourage "fighting among brothers in religion."
Soon after the assassination, a rocket slammed into the Palestine Hotel in Baghdad, blowing a large hole in the wall and devastating an unoccupied room. Police said three people were killed, but hotel staff said only two employees received minor injuries.
It appeared the rocket had been fired from a Shiite militia area at the Green Zone, located directly across the Tigris River from the hotel, but had strayed off-target.
Associated Press writers Sameer N. Yacoub, Hamid Ahmed and Qassim Abdul-Zahra in Baghdad contributed to this report.