( Reuter )- Iraq's U.S.-backed Prime Minister Nuri al-Maliki vowed on Thursday security forces would battle Shi'ite militia in Basra "to the end" despite thousands of protesters marching to demand his resignation.
Mehdi Army fighters loyal to cleric Moqtada al-Sadr remained in control of streets of Basra, Iraq's second biggest city and main oil hub, defying a three-day government offensive that has led to violence spreading across the south and Baghdad.
Saboteurs blew up one of Iraq's two main oil export pipelines from Basra, cutting at least a third of the exports from the city which provides 80 percent of government revenue.
It was the first time since 2004 that the southern supply route had been disrupted, and U.S. oil prices briefly rose more than $1 a barrel after the blast.
Maliki, who has traveled to Basra to oversee the crackdown, told tribal leaders it was sending "a message to all gangs that the state is in charge of the country."
"We entered this battle with determination and we will continue to the end. No retreat. No talks. No negotiations."
More than 130 people have been killed and hundreds wounded since the government launched its operation on Tuesday, exposing deep divisions between powerful factions within Iraq's majority Shi'ite community.
The clashes have all but wrecked a truce declared last August by Sadr, which Washington had said helped curb violence.
The government says it is fighting "outlaws," but Sadr's followers say political parties in Maliki's government are using military force to marginalize their rivals ahead of local elections due by October.
Tens of thousands of Sadr supporters marched in Baghdad in a massive show of force for the cleric, demanding Maliki's ouster. In the vast Sadr City slum named after the cleric's slain father crowds of angry men jammed the main circle chanting slogans.
"We demand the downfall of the Maliki government. It does not represent the people. It represents Bush and Cheney," marcher Hussein Abu Ali said.
The slum of 2 million people has been locked in a virtual state of siege.
"We are trapped in our homes with no water or electricity since yesterday. We can't bathe our children or wash our clothes," said a resident who gave his name as Mohammed.
Demonstrations were also held in the Kadhimiya and Shula districts, among the largest anti-government protests Maliki's government has faced although the total number of marchers was impossible to verify. An Interior Ministry source said hundreds of thousands took part.
A Reuters correspondent in Basra said Iraqi forces had cordoned off seven districts but were being repelled by Mehdi Army fighters inside them. Helicopters swooped overhead.
Reuters television pictures showed masked Mehdi fighters firing mortars and waving rocket launchers in the air while they danced with children in the empty streets. Some showed off captured government vehicles sprayed with Mehdi Army slogans.
Authorities imposed curfews in other Shi'ite towns to halt the spread of the violence.
A massive mortar bombardment struck the main riverside police base at Basra palace before noon on Thursday and heavy shooting broke out in a main commercial street in the city.
An Interior Ministry source said 51 people have been killed and more than 200 wounded so far in Basra alone. Basra's police chief survived a roadside bomb which killed three bodyguards.
Clashes have spread in the past two days to the southern cities of Kut, Hilla, Diwaniya, Amara and Kerbala, as well as several Shi'ite neighborhoods of Baghdad.
Forty-four people have been killed and 75 wounded in Wasit province, police chief Abdul Hanin al-Imara said. U.S. planes flew over the provincial capital Kut and gunfire rang out as troops entered the streets, a Reuters witness said.
Ali Bustan, head of the health directorate for eastern Baghdad, said 30 bodies and more than 200 wounded had been brought to two hospitals in Sadr City.
Reuters television pictures showed fighters in T-shirts and jeans firing rocket-propelled grenades and rifles on the streets of the Shaab district of northern Baghdad. Police said Sadr followers had set ablaze a building of Maliki's Dawa party in Shaab. Clashes had broken out in the southern al-Amin district.
U.S. and Iraqi checkpoints near Sadr City came under fire, said U.S. military spokesman Lieutenant-Colonel Steven Stover.
Gunmen burst into the home of a government security spokesman, set it on fire and kidnapped him.
Sadr's aides say his ceasefire is still formally in place. But his followers have staged a "civil disobedience" campaign, forcing schools and shops to shut, and Sadr has threatened to declare a "civil revolt" if the crackdown is not halted.
Mortar bombs and rockets have exploded across the capital for days. A mortar bomb at a Baghdad bus station killed three people and wounded 15. A strike near the U.S. embassy in the fortified Green Zone sent a column of black smoke into the sky.