Spokesman for al-Sadr's office in Kadhimiya, Shaikh Salah al-Ubaidi, told Aljazeera the troops detained Hazim al-Araji, who heads the office. They raided al-Araji's 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."
British soldiers have clashed
with al-Sadr fighters in Basra
Both officials had not received any warning prior to the raids, he added.
"The al-Sadr office has cooperated a great deal with Kadhimiya police" during events marking the anniversary of the death of a Shia Imam Musa bin Jaffar recently, he said.
The cooperation was maintained to ensure security and peace in the city and the results were positive, the spokesman added.
Shaikh Salah said al-Sadr's office believed the action was "an attempt to prevent al-Sadr movement from any political participation in the future".
In Basra on Saturday, a member of al-Sadr's al-Mahdi Army was killed during clashes with British forces in the area of al-Twaisa.
A British soldier was also seriously injured after al-Mahdi Army fighters opened fire at a patrol in the city, Captain Hisham Halawi, a spokesman for the British forces, told Aljazeera.
Meanwhile, a new round of talks to cease hostilities in the Baghdad slum of Sadr City have ended in deadlock, with al-Sadr's al-Mahdi Army fighters refusing American demands to disband and turn in their weapons, the US military and al-Sadr aides said on Sunday.
A group of tribal leaders and al-Sadr representatives met on Saturday a Baghdad city councilman acting as an intermediary between the two sides to discuss a 12-point proposal. The proposal calls for fighters to disarm in exchange for millions of dollars in reconstruction money and compensation for victims.
Al-Sadr's al-Mahdi army refuses
to disband and turn in weapons
The plan would also have Iraqi forces take over much of the security in the east Baghdad slum and limit the movement of US troops. American commanders did not take part in the talks, the military said.
"The only sticking point is that the Americans are insisting on disbanding the al-Mahdi Army and this is not possible for us," al-Sadr's chief representative at the meeting told The Associated Press.
Colonel Bryan Roberts, a spokesman for the 1st Cavalry Division, accused al-Sadr of reneging on past promises.
"Previously, Muqtada's militia agreed to turn in their heavy weapons and disband but there is no evidence they have complied," Roberts said in a statement.
"Their random acts of violence have killed innumerable innocent Iraqi civilians and will not be tolerated," he said. "Our forces stand firm: Muqtada's militia must disarm and disband."
Al-Sadr led a three-week uprising against American forces in the holy city of Najaf that ended last month with a peace deal allowing fighters to walk away with their guns.
Al-Sadr's chief representative said he wanted a similar deal for Sadr City.
"I do not know why the Americans are ready to accept compromises everywhere in Iraq except Sadr City," he said.