Seven American soldiers have been killed in Baghdad in the latest round of clashes between Shia Muslim militia and US-led occupying forces in Iraq.
Moqtada Sadr supporters want US-led forces out of Iraq
They died in a battle for control of public buildings in the Sadr City area that had been taken over by followers of a radical Shia cleric, Moqtada Sadr.
His supporters were involved in earlier clashes in Najaf that left about 20 Iraqis and two coalition soldiers dead.
Dozens of his supporters occupied the governor's office in Basra on Monday.
Moqtada Sadr is an increasingly outspoken opponent of the occupation.
Recent days have seen a number of demonstrations by his followers following the arrest of one of the cleric's top aides and the closure of his newspaper by the coalition, which said the publication was inciting anti-US violence.
Most previous attacks on coalition troops had been by fighters drawn from the Sunni Muslim minority - who received favourable treatment under former Iraqi leader Saddam Hussein - rather than the majority Shia.
The BBC's Barbara Plett in Baghdad says Mr Sadr's movement is generally seen as the most radical of the Shia religious movements.
She adds that a new front may have opened in the battle to pacify Iraq.
Two senior US congressmen have warned President George W Bush's administration that Iraq faces the possibility of civil war.
The Republican and Democratic Party leaders of the Senate Foreign Relations Committee, Richard Lugar and Joe Biden, said the US should consider postponing the handing over of sovereignty on 30 June.
Dozens of men loyal to Moqtada Sadr - many of them armed - occupied the governor's office in Basra in a dawn raid on Monday but apparently met no resistance, the BBC's Dumeetha Luthra reports from the roof of the building.
Policemen guarding the building appear to have taken sides with the protesters who claim they are engaged in a peaceful sit-in, our correspondent says, and there is no sign of the UK coalition forces who control Iraq's second city.
The fighting in Baghdad broke out after members of a militia loyal to Mr Sadr took control of police stations and government buildings in Sadr City, the US military said in a statement.
It said militiamen attacked the soldiers with small arms fire and rocket-propelled grenades in the heavily populated Shia area on the eastern outskirts of the capital.
As well as the seven troops killed, at least 24 were wounded while Iraqi casualties are unknown.
The Baghdad fighting came hours after a march by Mr Sadr's followers on a coalition force base near the holy city of Najaf ended in violence.
The protest took place outside Najaf's Spanish garrison where troops from El Salvador and other Spanish-speaking countries are also based.
It is unclear who fired first but a coalition soldier from El Salvador and one from the US were killed along with about 20 Iraqis. Many more were injured.
'Terrorise your enemy'
Following the Najaf violence, a spokesman for Moqtada Sadr said the cleric had called for an end to protests, asking his supporters instead to gather at his offices or in mosques.
"Terrorise your enemy, as we cannot remain silent over its violations," a translation of his statement said.
It was not clear whether this was an order for his followers to resort to violence.
The US administrator in Iraq, Paul Bremer, said protesters had "crossed the line and moved to violence".
"This will not be tolerated", he added.
In a separate incident on Sunday, the US military said two marines had been killed in the province of Al-Anbar, a hotbed of anti-coalition violence.