A spokesman for al-Sadr's office in al-Kadhimiya, Shaikh Salah al-Ubaidi, told Aljazeera Iraqi troops had detained Shaikh al-Aaraji, who heads the office.
Al-Sadr aide Shaikh Hazim was
detained by Iraqi troops on Sunday
They raided his home in the early hours of Sunday and detained him and his brother.
The troops also raided the home of a neighbour of Shaikh Raid al-Kadhimi, who is al-Sadr's official representative at Friday sermons in the city.
They raided the home mistakenly, believing it was al-Khadimi's, the spokesman said, adding that the raids were "violent".
"They fired stun grenades and opened heavy fire," Shaikh Salah said. "They even hit children and damaged contents of the homes."
Shaikh Salah said the Shia leader's office believed the action was "an attempt to prevent al-Sadr movement from any political participation in the future".
The brother of the detained Shaikh al-Aaraji has told Aljazeera that lately there have been serious attempts to transform al-Sadr movement "into a political body".
Baha al-Aaraji said his brother had told him the day before his arrest he "was almost finished with this project and was supposed to be announced in the next few days".
"If the Americans and their allies in Iraq think that arresting or eliminating leaders of the al-Sadr movement will end it (political involvement), then they are wrong"
brother of detained Shaikh Hazim al-Aaraji
"However, this project builds fears in the Americans and parties participating in the authority from extending this public base."
"If it turn into a political body, it would ruin the plans of these mercenary parties, the Americans and the Zionist lobby in Iraq," Baha al-Aaraji added.
"That is why they arrested Hazim who was supposed to announce this project as entrusted by our leader and particularly that some sides in the government and in its ally parties already knew about this project."
He added: "what they fear more is the collaboration between al-Sadr movement and other prominent Islamic parties".
Baha al-Aaraji said the arrest meant that US forces did not want the political process in Iraq to be planned and organised by Iraqis.
"They want a political process that serves their interests," he told Aljazeera.
"If the Americans and their allies in Iraq think that arresting or eliminating leaders of al-Sadr movement will end it, then they are wrong."