Heavy fighting has been raging in Basra following a major pre-dawn offensive by thousands of Iraqi troops against Shi'ite militias in the southern city.  Iraqi officials say at least 12 people have been killed and 18 wounded in the clashes, but the numbers could be higher.  Shi'ite cleric Moqtada al-Sadr is threatening a campaign of "civil disobedience" if the crackdown on his militias continues.  VOA Correspondent Challiss McDonough has more from our Middle East bureau in Cairo.

Iraqi security forces and Shi'ite militias fought fierce gun battles on the streets of Basra after Iraqi troops launched a pre-dawn offensive to crack down on armed groups in the southern city.

Residents of Basra said they could hear explosions and automatic weapons fire throughout the day, and columns of black smoke were seen rising above the city.

Iraqi Prime Minister Nouri Al-Maliki went to Basra on Monday to personally supervise the security operation, which targeted districts in central and northern parts of the city, strongholds of the Mehdi Army militia led by Shi'ite cleric Moqtada Al-Sadr.

The prime minister issued a statement saying the offensive was targeting "outlaws" in Basra and was designed to bring the southern city under government control.

But Sadrist lawmakers and officials denounced the offensive and said they felt the government is targeting the Sadr organization, which is a powerful political force in southern Iraq.

The cleric's aide Hazem Al-Aaraji read a statement on behalf of Sadr, demanding and end to the operation.

He said Sadr's group was calling for a nationwide strike, and then if the Iraqi government does not comply, he said, "the second step will be civil disobedience in Baghdad and other provinces."  He said after that would come a "third step," but did not say what it would be.

Moqtada al-Sadr recently renewed a cease-fire that he originally ordered in August, instructing his powerful militia not to attack U.S. or Iraqi security forces.  The cease-fire is seen as a major contributor to the reduction in violence in Iraq over the last year.

U.S. and Iraqi officials say they are not interested in fighting the overall Sadr organization, only what they call "rogue elements" who are refusing to obey the cease-fire.

British troops handed control of Basra province over to Iraqi security forces in December and were not taking part in the operation, dubbed "Charge of the Knights."

Clashes were also reported in the city of Kut.  Overnight curfews were being imposed elsewhere in central and southern Iraq, including Kut and Nasiriya.

Angry protesters took to the streets in the Sadr City district of Baghdad, a sprawling slum that is a Mehdi Army stronghold.  Shops and businesses were reported to be closed throughout the neighborhood, with some residents saying they were too afraid of unrest to leave their homes.