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.