BAGHDAD — The militant Shiite cleric Muqtada al-Sadr called yesterday for followers to hold weekly protests against an American-Iraqi security deal under negotiation that could lead to a long-term American troop presence.

The outcry by Mr. Sadr could sharply heighten tensions over the proposed pact, which is supposed to be finished by July to replace the current U.N. mandate overseeing America-led troops in Iraq.

Mr. Sadr — whose powerful Mahdi Army militia has often battled American and Iraqi forces — is one of the most vocal opponents of the American presence in Iraq, but many Iraqis have expressed worries over any final deal that involves permanent American bases.

Mr. Sadr, who is believed to be in Iran, did not give specific guidance on the planned demonstrations in a statement issued by top Shiite religious officials. Any major marches, however, could put added strain on a tenuous truce between the Mahdi Army and the government of Prime Minister al-Maliki after weeks of battles that began in late March.

In northern Iraq, meanwhile, a car bomb exploded near a popular market in Tal Afar, killing four civilians and wounding 46 others, the city’s mayor, Major General Najim Abdullah, said.

There was no immediate claim of responsibility. It came hours after an Al Qaeda in Iraq front group warned that insurgents would retaliate against American and Iraqi forces, which began a crackdown nearly two weeks ago in the main northern city of Mosul, 40 miles east of Tal Afar.

A man claiming to be a spokesman for the Islamic State of Iraq in Ninevah province, which includes Mosul, said in a videotape posted online that insurgents were at “full strength” despite the Mosul sweeps and were just waiting for the proper time to counterattack.