In a critical test for the Shia-led government and its US-trained forces after two days of killings between Sunnis and Shias, police and Iraqi soldiers were deployed in force, turning back motorists unaware of the ban announced overnight.

But in the sprawling Shia slum of Sadr City, the streets teemed with people who defied the curfew after radical Shia cleric Moqtada al-Sadr called on his Mehdi Army militia to attend Friday prayers but banned attacks on Sunni Muslims.

"We are not enemies but brothers," Sadr said in a statement.

"Anyone who attacks a Muslim is not a Muslim and he who assaults sacraments and mosques shall get his just punishment."

Reprisal attacks on dozens of minority Sunni mosques and about 200 killed around Baghdad alone in just over 48 hours following Wednesday's suspected al-Qaeda bombing of a Shia shrine have seen the United States and United Nations joining efforts to avert all-out civil war that could wreck US hopes of withdrawing troops and inflame the entire Middle East.

Washington and the Shia-led Iraqi government have blamed al-Qaeda's Sunni militants for the destruction of the Golden Mosque in Samarra, saying they are seeking to spark just such a conflict. A militant grouping led by al-Qaeda accused Shias of conducting the bloodless bombing to justify reprisals.

In southern Baghdad on Friday, Shia militiamen clashed with gunmen, leaving Iraqi security forces who are trying to enforce a curfew helpless to stop them.

Police sources said the clashes were between unidentified gunmen, possibly minority Sunnis, and members of the Mehdi Army militia. It was not immediately clear if anyone was hurt in the skirmishes in the Saidiya area.

Twenty bodies of people who were killed overnight and this morning have been brought to the morgue, a police source said.

Even if the curfew calms passions on the Muslim holy day, Iraq's government will still have to demonstrate it can control Shia militiamen who have been attacking Sunni targets and setting up their own checkpoints in defiance of the state.

Following the government's decision to extend an overnight curfew until 4 p.m. (1300 GMT), the large crowds at Sadr City posed a challenge to Iraq's security forces, many of whom are drawn from militia groups they may be ordered to control.

About 20,000 Sadr supporters crowded Sadr City chanting slogans for Shia-Sunni brotherhood; in one central Baghdad Sunni mosque, only 10 people showed up to pray instead of the usual 200.