Also Friday, Iraqi police say at least 16 people have been killed and 20 wounded when a car bomb struck a market in the northern city of Tal Afar.
A senior police official in the nearby city of Mosul says the car was parked when it exploded by the market, crowded with shoppers.
The official spoke on condition of anonymity because he wasn't authorized to release the information. The bombing is the latest in a series of deadly attacks by suspected insurgents seeking to chip away at recent security gains.
Al-Sadr's statement - read to worshippers during Friday prayers in Baghdad's former militia stronghold of Sadr City - is in line with details revealed earlier this week and appears to be an extension of plans he announced in June aimed at asserting more control over the militia.
"Weapons are to be exclusively in the hands of one group, the resistance group," while another group called Momahidoun is to focus on social, religious and community work, Sadrist cleric Mudhafar al-Moussawi said.
He said the announcement was particularly aimed at members of al-Sadr's Mahdi Army militia, which has been blamed for some of the worst violence against American troops and rival Sunni Arabs.
Thousands of worshippers streamed out into the streets after the Islamic service, burning an American flag and shouting: "No, no to America. No, no to occupation."
The cleric has linked the reorganization of the Mahdi Army to U.S.-Iraqi negotiations over a long-term agreement that would extend the American presence in Iraq after a U.N. mandate expires at the end of the year. Al-Sadr and his followers want the deal to include a timeframe for an American withdrawal and have warned they may not suspend operations without such a clause.
Several cease-fires by al-Sadr have been key to a sharp decline in violence over the past year, along with a Sunni revolt against al Qaeda in Iraq and a U.S. troop buildup. But American officials still consider his militiamen a threat and have backed the Iraqi military in operations to try to oust them from their power bases in Baghdad and elsewhere in Iraq.
The fighting cells will be "small and limited" and will only launch attacks under direct orders from al-Sadr in case of "dire necessity," the cleric's spokesman Sheik Salah al-Obeidi told The Associated Press in the holy city of Najaf.
He also ruled out attacks on Iraqis and claimed Mahdi Army members had shown interest in making the program a success.
"Now our stance is to watch the political developments and the security agreement. We will see if there will be a withdrawal timetable or not. We will wait for the results. These cells have not yet conducted any operations," he added.
Two Iraqi officials close to Prime Minister Nouri al-Maliki have said government and U.S. negotiators on all American combat troops leaving Iraq by October 2010, with the last soldiers out three years after that. The officials all spoke on condition of anonymity because the talks are ongoing.
U.S. officials, however, insisted no dates had been agreed.
"It's premature to say what the aspiration goals and time horizons are going to be," and a date for troop withdrawals will not be "plucked out of thin air," White House press secretary Dana Perino said, speaking to reporters in Beijing on Friday where U.S. President George W. Bush is attending the Olympics.
Throughout the conflict, Mr. Bush steadfastly refused to accept any timetable for bringing U.S. troops home. Last month, however, Mr. Bush and al-Maliki agreed to set a "general time horizon" for ending the U.S. mission.
Both Iraqi and American officials agreed that the deal is not final and that a major unresolved issue is the U.S. demand for immunity for U.S. soldiers from prosecution under Iraqi law.
In northern Iraq, Kurdish leader Massoud al-Barzani visited the disputed city of Kirkuk and called for rival Kurds, Turkomen and Sunni Arabs "to have an open dialogue" to resolve their disagreement over sharing control of the oil-rich city.
His appeal came two days after the issue blocked passage of a provincial elections law, casting doubt whether U.S.-backed balloting can be held this year in the country's 18 provinces.
The bill failed because the sides were unable to come to terms on a power-sharing deal for the multiethnic region around the city of Kirkuk, the center of Iraq's northern oil fields.
Kurds consider Kirkuk their ancestral capital and want to incorporate it into their self-ruled region in the north. Most Arabs and Turkomen want Kirkuk to remain under central government control.
In Washington, the State Department expressed irritation that the parliament had gone into summer recess without having reached a compromise on the matter.
"The status of Kirkuk is indeed a sensitive issue that needs to be addressed in a serious fashion, but it is an issue that cannot be solved through the legislative mechanism of the election law," spokesman Gonzalo Gallegos said. "The election law should not be held hostage to that problem."