The word went out on April 6 at noon, with the blast of the call to prayer: American soldiers had raided an office of Moqtada al-Sadr, the radical Shiite cleric, and torn up a poster of his father, one of Iraq’s most revered martyrs.

The Khadamiya bazaar exploded in a frenzy. Shopkeepers reached beneath stacks of sandals for Kalashnikov rifles. Boys wrapped their faces in black cloth. Men raced through the streets, kicking over crates and setting up barriers. Some handed out grenades. Within minutes this entire Shiite neighborhood in central Baghdad had mobilized for war.

“We’re going to attack a tank!” yelled Majid Hamid, 32, waving an assault rifle.

The incident was another example of the power vacuum spreading across Iraq—during the disturbance in Khadamiya, there were no American soldiers, no Iraqi police and no order. It also cut to the heart of the militia issue, which remains a problem despite the occupation authorities’ insistence that private armies disband. And it showed the depth of support for Mr. Sadr, the firebrand cleric who is blamed for the most serious insurrection yet and is now wanted by the Americans.

American officials estimate the number of people in his private army at 3,000. But as the display of force on April 6 showed, there were thousands of men and boys in just one Baghdad neighborhood ready to fight for Mr. Sadr. And as battles raged throughout the country, in Sunni bastions like Falluja and Ramadi and in Shiite areas like Sadr City, it was growing increasingly clear that the militias could materialize almost instantaneously. While many people—bakers, teachers, sandwich makers—hold normal jobs, when the call comes, they line up with Mr. Sadr’s force, the Mahdi Army.

“This man is not a firefighter,” said Lt. Mohammed Abu Kadar, tapping one of his men on the shoulder outside a fire station in Khadamiya. “He is Mahdi Army.”

“This man, too,” the lieutenant, a two-star officer of the Iraqi Civil Defense Corps, said, grabbing another firefighter. “He may wear this uniform, but he is Mahdi Army.”

Then the lieutenant tapped his own chest. “We may work for the government now,” Lieutenant Kadar said. “But if anything happens, we all work for Sadr.”

The situation in Khadamiya is similar to the one in Kufa, a small town south of the capital, which is entirely controlled by Mr. Sadr’s militia, and Sadr City, a predominantly Shiite neighborhood of two million in Baghdad, where Mr. Sadr’s men have driven out the Iraqi police in clashes that have killed eight American soldiers.

There may also be an ominous synergy developing between Sunni and Shiite insurgents. On Monday, insurgents fought a gun battle against United States troops in a Sunni neighborhood near Khadamiya in which three soldiers were killed. Witnesses said the attackers included a mix of Shiites and Sunnis. “There were Shiites from Sadr City and mujahedeen from Falluja,” a hotbed of Sunni resistance, said Ayad Karim, a shopkeeper. “Now the resistance is united.”

On a white sheet hung from the bullet-ridden walls of a Sunni mosque were the words: “Our banner in Adamiya is the same banner as in Khadamiya. If they have a problem, we are their backup and their right hand.”

Adamiya is a mostly Sunni area. Khadamiya is mostly Shiite. The two neighborhoods are linked by a bridge over the Tigris River. Rival Sunni and Shiite gangs used to cross the bridge to rumble. Now, people say, militants cross the bridge to coordinate attacks.

According to witnesses, the disturbance on Tuesday started when American soldiers raided a Khadamiya office of Mr. Sadr’s, looking for weapons. Jaffar Qasim, a 29-year-old guard, said the soldiers kicked away the lunch he was eating and then ripped off the wall a poster of Mr. Sadr’s father, who was assassinated in 1999. Hours later, Mr. Qasim was still crying. His hands vibrated with frustration. The American soldiers, he said, also stomped into a prayer room where shoes are forbidden.

“If I could kill them I would,” he said, looking at the dusty footprints of combat boots on a worn red carpet. “But I had my orders. And I didn’t have a gun.”

American military officials would not comment on the raid or the activity in Khadamiya except to confirm that three soldiers were killed there during attacks.

Many of Mr. Sadr’s followers said they had made an agreement with American commanders that they would avoid the bloodshed that erupted in Sadr City on Sunday if the American forces agreed to stay out of the neighborhood, home to a famous Shiite shrine. But the raid broke that agreement, Mr. Sadr’s followers said. And destroying the poster of one of their martyrs seemed the ultimate disrespect.

So the revolt began. Men in Khadamiya’s bazaar suddenly produced guns. Shopkeepers dashed into the streets with rifles. “Moqtada! Moqtada! Moqtada!” many yelled.

Almost overnight, seemingly, Mr. Sadr, 31, has emerged as the most dangerous man to the American-led occupation. But if the increasing attendance at his demonstrations is an indication, his support has been building for months. Part of his appeal is his militancy. While other Shiite clerics have pressed for moderation, Mr. Sadr has openly called for an end to the occupation. His newspaper, Hawza, was closed last week after American authorities accused it of printing lies that incited violence. That began a cycle of protests that culminated in widespread bloodshed on Sunday.

Months ago, some of Mr. Sadr’s rivals said he had only a few hundred armed men behind him. Hazim al-Aarji, Mr. Sadr’s chief commander in Khadamiya, maintains there are 50,000 members of the Mahdi Army just in Baghdad. Mohammed Kadem, a 23-year-old Mahdi Army fighter from Khadamiya, said the force had been lying quietly for months, with arms they looted nearly a year ago. “We just kept them in our homes,” Mr. Kadem said. “We knew this time might come.”

Mr. Kadem detailed a training program in which he and other Shiite youths took buses to Sadr City to practice marksmanship in an open field. Marching orders are disseminating through mosques, Mr. Kadem explained, and ammunition is supplied by central offices.

On Tuesday, Mr. Kadem was part of a crowd massing in the streets of Khadamiya, screaming and staring down American tanks. A few gunshots were exchanged and there were different reports of casualties, but nothing could be confirmed on the night of April 6. Next to him was Mr. Qasim, the guard who saw the poster ripped down.

This time Mr. Qasim was no longer crying. And this time, Mr. Qasim had a Kalashnikov in his hands.

---

—The New York Times, April 7, 2004