BAGHDAD, Iraq — Shiite Muslim cleric Muqtada al Sadr threatened yesterday to formally end an already tattered cease-fire he had ordered his Mahdi Army militia to obey. The radical preacher also canceled plans for a massive protest march against the Iraqi government and its American allies scheduled for today, after complaining about harassment from the authorities.

The warning on the truce came on the day that U.S. Army General David Petraeus and Ambassador Ryan Crocker testified to Congress about the situation in Iraq and as American-led forces continued to battle militants in Baghdad.

“The Iraqi government should know that the Mahdi Army will stand shoulder to shoulder with the Iraqi people to provide all they need from security, stability and independence,” Mr. Sadr said in a statement. “If this means that the ‘freeze’ shall be lifted in order to achieve our aims and religious beliefs, then we shall do so later in an announcement.”

Mr. Sadr had called for a million followers to descend on the capital in protest today to mark the fifth anniversary of Baghdad’s fall to American troops. But Mr. Sadr spokesman Salih Obeidi complained yesterday that the government had blocked the movement from bringing supporters to Baghdad from southern Iraq.

“The government made many obstacles against us, so we couldn’t go on with it,” Mr. Obeidi said of the march. He added that Mr. Sadr had made his decision to protect his followers from any provocations on the government’s side.

There had been concerns that the march would aggravate tensions between Mr. Sadr and Prime Minister Maliki of Iraqi and spur further violence. Despite Mr. Sadr’s announcement, the government announced an expected ban on vehicle traffic in the capital for today, starting at 5 a.m.

Yesterday afternoon, the streets were near empty at the roads into Sadr City, as a few people were seen walking with bags of groceries into the neighborhood. The mostly Shiite area, home to 2.5 million people, has been a battle zone since the end of last month. Residents reported gun battles in the neighborhood yesterday. Police in Sadr City reported 12 deaths in the latest violence, including a mortar attack and an airstrike by American forces. Hospital officials reported that at least 47 wounded Iraqis were treated. The American military confirmed it had launched Hellfire missiles that killed as many as 12 people it said had been firing mortars and rockets.