Tens of thousands of Shia demonstrators rallied to a call by Moqtada al-Sadr, the anti-American cleric, to protest against coalition forces in Iraq yesterday, the fourth anniversary of the fall of Baghdad.

The vast gathering of men burning US flags, carrying giant Iraqi ones and waving banners denouncing President Bush was a huge show of strength for the firebrand cleric. Policemen stayed in their bases and armed militiamen from Hojetoleslam al-Sadr’s al-Mahdi Army enforced security.

But behind such displays of unity, many point to fault lines developing in the militia, estimated to number 20,000 gun-men. With its leaders in exile — Hojatoleslam al-Sadr is believed to be in Iran — hiding or in US custody, many militiamen are increasingly frustrated. Iran is thought to be fishing for rogue elements to bring under its influence.

“Iran is afraid of the Americans now and wants to make sure the Mahdi Army can control Iraq and beat the Americans,” said Abu Bakr, an al-Mahdi commander in Baghdad. A Western diplomat in Iraq said that there were splits within the militia and the political movement allied to it. “Every week that goes by, and we’ve been seeing this since November, there’s another example of how fractured it is,” he said.

Al-Mahdi fighters were ordered to stand down at the start of the US security surge in Baghdad, to avoid being blamed should it fail. But some are champing at the bit to fight the “occupiers”. In Diwaniyah, a Shia town in central Iraq, al-Madhi forces clashed with US and Iraqi troops before their leader ordered them to allow Iraqi forces to take control.

Some see the cracks as a positive sign that parts of the Sadrist movement, a keystone of the Government, may be looking for a more constructive political role. Others worry that if it splits, the Government will face an unpredictable foe.

Developed from an armed militia to protect poor Shia areas after the US-led invasion, the al-Mahdi Army became an influential force. It may now have become a victim of its own success, a sprawling mob that often operates at the whim of local commanders. “There are at least three elements,” said the diplomat. “The criminals and opportunists, those close to Moqtada and those with an Iranian agenda.”

Since last summer, the militia has became associated with death squads terrorising the capital, but has tried to distance itself from the killers and kidnappers. Abu Haidar, an al-Mahdi fighter, said that the movement had an intelligence service to find criminals and Sunni “terrorists”. Thirty al-Mahdi members had been executed in the past five months for abusing their positions.

Despite orders to halt operations, Abu Haidar said that al-Mahdi hit squads were still abducting suspected Sunni extremists. Before the latest US expansion in Baghdad, dozens of bodies were dumped on the streets every day and al-Mahdi militiamen passed easily through checkpoints, where soldiers were complicit or cowed.

In the boots of their cars were victims heading for Sharia tribunals or corpses on the way to the river. “Our guys buy cars according to the size of the boot. The Toyota Crown Super saloon is a favourite,” Abu Haidar said. “You can get four people in the boot.”

The number of corpses has dropped, but many Shia complain of attacks by Sunni extremists. Unless US and Iraqi troops contain the violence, the militia may return, but with even more popular backing.

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