At least 12 people have been killed in heavy fighting between Shia militias and Iraqi security forces in the southern Iraqi city of Basra.
The fighting comes as part of a major operation to bring the city under government control.
The British handed control of Basra back to the Iraqis in December.
Television pictures showed Iraqi troops running through the streets firing weapons.
Witnesses said the streets were empty aside from the security forces, emergency vehicles and people in cars fleeing the fighting. Shops and markets were closed.
Meanwhile, police have imposed curfews in three southern Iraqi Shia cities following the clashes between in Basra.
The curfews in Kut and Samawa are indefinite while Nasiriyah's restrictions apply until tomorrow morning.
The curfew in Kut was imposed after clashes in the city, while in Samawa and Nasiriyah it was a precautionary step, a security official said.
Earlier, Shia cleric Moqtada al-Sadr called on Iraqis to stage sit-ins and threatened a countrywide civil revolt if attacks by US and Iraqi security forces continue against his followers.
He also threatened a third step, but said it was too early to announce what that would be.
