Salah al-Obeidi, a spokesman for al-Sadr who delivers Friday prayers on behalf of the cleric in the nearby city of Kufa, and Muayed al-Khazraji, a leading aide from Baghdad, were taken in a raid by U.S. and Iraqi forces at about 1 a.m. local time, said Muhammed Jamil, a spokesman in the city some 100 miles south of Baghdad.
The incident was not immediately confirmed by U.S. forces.
Jamil said the cleric called on the followers of his Mahdi Army not to retaliate.
"Muqtada al-Sadr called on Jaish al-Mahdi (the Mahdi Army) to exercise calm and self control and not to be dragged into the American scheme to drive them into a new confrontation."
In another incident, a joint American and Iraqi patrol clashed with Mahdi Army forces in Diwaniyah, 80 miles south of Baghdad, in a raid to arrest a Mahdi Army leader, police said.
Two Iraqi soldiers were wounded and four injured, said police Capt. Abbas al-Bayati. An Iraqi armored personnel vehicle and a U.S. Humvee were damaged, but there were no immediate reports of American casualties, al-Bayati said.
Al-Bayati said he was still trying to confirm whether there were any Mahdi Army dead or wounded.