By Aref Mohammed in BasraTHE death toll from clashes between Iraqi security forces and the Mehdi Army militia in Basra has risen to 12, and violence appears to be spreading to Baghdad and other cities.
Police and health workers said at least 12 people were killed in the fighting in districts of central and northern Basra where Shiite cleric Moqtada al-Sadr's Mehdi Army has a strong presence.
"There are clashes in the streets. Bullets are coming from everywhere and we can hear the sound of rocket explosions. This has been going on since dawn," resident Jamil said.
Columns of black smoke rose above the city and explosions and machinegun fire could be heard. Reuters Television pictures showed masked gunmen firing mortars in the street, while others drove around in captured Iraqi army and police vehicles.
The Mehdi Army, which has thousands of fighters, has kept a relatively low profile since last August when Sadr called a ceasefire, one of the main factors behind the sharp reduction in sectarian violence in Iraq in recent months.
But the militia has chafed at the truce, saying US and Iraqi forces exploited it to carry out indiscriminate arrests.
In a statement read out by a senior aide on Tuesday, Sadr called on Iraqis to stage sit-ins all over Iraq and said he would declare a "civil revolt" if attacks by US and Iraqi security forces continued.
He also threatened a "third step", but said it was to early to announce what it would be.
Sadr's followers launched what they called "a civil disobedience campaign" in Baghdad on Monday, forcing store-owners to close in several districts.
Pro-Sadr students forced Mustansiriya University in Baghdad to close on Tuesday. Members of Sadr's movement said the protest would spread to other towns and cities from Wednesday.
Police sources said Sadr supporters seized control of five districts in the southern town of Kut on Tuesday after clashes between gunmen and police.
In Baghdad, US and Iraqi forces sealed off the Mehdi Army stronghold of Sadr City, a sprawling slum of 2 million people, after the militia ordered police and soldiers off the streets.
Police said fighting erupted in several Sadr City neighbourhoods between Mehdi Army fighters and the Badr Organisation, the armed wing of a rival Shiite faction.
Baghdad's Green Zone, the government and diplomatic compound, was hit by several salvoes of rockets during the day. US military spokesman Lieutenant-Colonel Steven Stover said they had been fired from Sadr City.
Police imposed curfews in the southern towns of Kut, Hilla and Samawa, capital of Muthanna province.
In Basra two ambulance drivers said they had transported eight bodies to Basra's Sadr Education hospital. A police major at al-Mawana hospital said four bodies were received.
"This operation will not come to an end in Basra without the law prevailing and being respected," Iraqi government spokesman Ali al-Dabbagh said.
But analysts said the government of Prime Minister Nuri al-Maliki, who was in Basra to oversee the operation, would struggle to overcome militias who were looking to keep hold of their share of Basra's oil wealth.
Sadrists and the Supreme Islamic Iraqi Council (SIIC), the two most powerful Shiite factions in Iraq, have been vying for control of Basra along with a smaller Shiite party, Fadhila, which controls key oil industry jobs in Basra.
Peter Harling, a Damascus-based analyst at the International Crisis Group think tank, said Sadr's followers were angry because they believed the US had chosen to support SIIC's Badr Organisation.
"The fact that Sadr called upon his followers to implement a civil disobedience campaign reflects the pressure building upon him. There is huge frustration among the group's rank and file."
Basra's oilfields hold 80 per cent of Iraq's oil wealth. Iraqi oil industry sources said the fields, which exported 1.54 million barrels of oil per day in February, were operating normally on Tuesday.
The British military said no British ground forces were involved in the operation, but warplanes from the US-led coalition were carrying out aerial surveillance.
Iraqi security forces took control of Basra from British forces in December.