Thousands of worshippers gathered to hear a sermon by Sheik Mohanned al-Mousawi, an al-Sadr ally who complained about arrests of the cleric's supporters by U.S.-backed Iraqi forces. But al-Mousawi urged restraint in keeping with a truce that ended fighting in Sadr City in May after a crackdown by the Iraqi military.
"We call on all brothers to be up to the responsibility and be committed to our leader's orders," al-Mousawi said.
Al-Sadr has lived in Iran for more than a year, deepening the uncertainty over the future of his militia, the Mahdi Army.
On Thursday, gunmen killed a mid-level aide to al-Sadr in the New Baghdad district in the capital, said a police official and an official with the cleric's organization.
Sheik Safaa al-Lami was shot in his car about a mile from the district's police station, where he had gone to ask about a detained member of al-Sadr's movement, the officials said. They spoke on condition of anonymity because they are not authorized to talk to the media.
The assailants used a pistol with a silencer to shoot al-Lami from a speeding car, the officials said. A friend who was with al-Lami escaped unharmed.
Also Friday, the White House said that the United States and Iraq have agreed to seek "a general time horizon" for deeper reductions in American combat troops in Iraq.
Iraqi officials, in a sign of growing confidence as violence decreases, have pressed the United States to agree to a specific timeline to withdraw U.S. forces. President Bush has opposed a timeline, and the White House said the timeframe being discussed would not be "an arbitrary date for withdrawal."
