They came at dawn, thousands of Iraqi troops and US special forces on a mission to reclaim a lawless city from the militias who ran it.

By the end of the day, al-Amarah was under Iraqi Government control - without a shot being fired.

The city had been taken over by the Shia al-Mahdi Army two years ago after British troops handed it to an ill-prepared Iraqi Army. “We can't say al-Amarah was entirely bad, there are good people here, poor people. But the city was controlled by the al-Mahdi Army, and these people are all backed by Iran,” said Captain Hussein Ali of the Scorpion police brigade, one of the Iraqi units drafted in to take part in Operation Omen of Peace.

Yesterday the city's streets - unpaved, dirt tracks between grubby, low brick houses - were crawling with Iraqi security forces. Soldiers searched houses as police manned checkpoints and Soviet-era tanks guarded bridges over the Tigris River.

The flood of troops, who had moved into position outside the city a week ago, had encountered no resistance as they moved in yesterday. The leaders of the Shia militias that once ruled as crime bosses and warlords were either gone or in hiding. Even the police chief fled a week ago, fearing arrest for his affiliation to al-Mahdi Army, while the mayor, a member of the Sadrist movement, was arrested.

Outside one of the long-neglected police bases built by the British Army, scores crowded to sign up as police officers, the only regular job in a city whose main industry is weapons smuggling from nearby Iran, but a profession that until now was closed to most.

“In the past, you needed contacts with the tribal sheikhs or to pay three million dinars in bribes to get a job as a policeman,” said Raed Mijbil, 30. “All the Iraqi security forces were corrupt.”

Nouri al-Maliki, the Shia Prime Minister, has insisted that his large-scale operations in the south are not targeting the Sadrist movement, which has been increasingly weakened by internal divisions, its brutal reputation for murder and extortion, and a more confident Iraqi military. Hojatoleslam Moqtada al-Sadr, the fundamentalist Shia cleric who heads the al-Mahdi Army and the Sadrist political movement, ordered his men not to resist the government forces, and a senior member of his parliamentary block expressed grudging support.

“We stand with the Government on imposing the law and we are showing goodwill,” said Bahaa al-Araji, a member of the Sadrist parliamentary bloc. “But law must be imposed on everybody. We hope the target of the plan is not our movement.”

Locals said that militiamen had been spotted throwing their weapons into the Tigris or trying to hide them along the lush river banks. One man said that he saw two women digging up a stash hidden by a fighter and taking them into a weapons collection point in the hope of a reward.

The ease with which Iraqi forces retook al-Amarah, for long a no-go zone, was in marked contrast to the battle for Basra launched by Mr al-Maliki in March. That conflict ended only when the Government cut a deal with Hojatoleslam al-Sadr, a ceasefire brokered by Iran.

“This way is better,” said Captain Ali, noting that an army battalion would stay in al-Amarah once the operation is finished clearing out the militias. “We don't want to lose people, and in urban warfare women and children can get killed.”

While the Prime Minister had personally to lead his shaky forces on the offensive in Basra, he and his army have gained in confidence since establishing control of the southern port city, even flooding the Sadr City stronghold in Baghdad with thousands of soldiers. For the first time in years the young cleric looks unsure of himself. Last week he announced that the main wing of al-Mahdi Army would devote itself to civilian projects, while a streamlined, smaller group would carry on attacking the US military, whom the demagogue deems a legitimate target for resistance.

Nabil Ibrahim, 20, an al-Amarah resident, was pleased to see the influx of government troops but upset that the men who had turned his city into a lawless no man's land had escaped. “The leaders who escaped aren't all al-Mahdi Army, they are Iranian intelligence agents. We are sad because they got away and they'll be back.”

Captain Ali denied that the criminal leaders had been allowed to get away. “We didn't just let them escape, this was a kind of amnesty. This was a last chance for those who were misled by the militias and regretted it,” he said. He said that the local population was co-operating with the security sweep, and that the army had found more than 900 roadside bombs in weapons stashes.

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