In a major show of force, British (search) soldiers used tanks to break down the walls of the central jail in this southern city late Monday and freed two Britons, allegedly undercover commandos, who had been arrested on charges of shooting two Iraqi policemen.

About 150 Iraqi prisoners also fled as British commandos stormed inside and rescued their comrades, said Aquil Jabbar (search), an Iraqi television cameraman who lives across the street from the jail. Earlier Monday, demonstrators hurled stones and Molotov cocktails at British tanks, and at least four people were killed.

The fighting in the oil city of Basra (search), 340 miles south of the capital, erupted after British armor encircled the jail where the two Britons were being held. During the melee one British soldier could be seen in a photograph scrambling for his life from a burning tank and the rock-throwing mob.

Arab satellite television stations showed pictures of two Western men sitting on the floor of the jail building with their hands tied behind their backs.

One of the men had a bandage covering most of the top of his head, the other had blood on his clothes. Television commentary identified them only as Britons.

British military officials had declined to comment on reports the two arrested men were soldiers operating undercover, but the Ministry of Defense in London told Britain's Press Association that "two military personnel were detained by Iraqi authorities earlier today."

British forces in Iraq are based in Basra and responsible for the city's security.

To the north, an estimated 3 million pilgrims — some carrying signs reading "We welcome martyrdom" — jammed the holy city of Karbala (search) for a major Shiite festival Monday in defiance of insurgent declarations of all-out sectarian war.

And an Iraqi court in Baghdad sentenced one of Saddam Hussein's nephews to life in prison for funding the country's violent insurgency and bomb-making after a previously unannounced trial. It was the first known trial of any of the former leader's family.

Elsewhere Monday, militants continued bloody attacks, killing 24 police and civilians and wounding 28 others.

But there were no attacks in Karbala, where security was so tight that authorities had banned vehicles from entering for several days before the holiday. Pilgrims were forced to pass through seven checkpoints inside the city before reaching holy shrines. About 6,000 policemen and Iraqi army troops were deployed in and near Karbala, and two leading Shiite militias provided additional security around the shrines of Imam Hussein and Imam Abbas.

As the festival continued, Al Aqaeda (search) in Iraq leader Abu Musab al-Zarqawi (search) purportedly issued a new vow, promising he would not attack followers of radical Shiite cleric Muqtada al-Sadr and other Shiite leaders opposed to Iraq's U.S.-backed government.

Last Wednesday, after insurgent forces were routed from their stronghold in the northern city of Tal Afar, al-Zarqawi, a Jordanian-born Sunni Arab, declared all-out war on Iraq's majority Shiites.

In an Internet statement which appeared Monday night on a Web site known for carrying extremist Islamist material, al-Zarqawi now appeared set on trying to split the Shiite community.

"Any Shiite group that condemns the government's crimes against the Sunnis in Tal Afar, and which doesn't provide help to the occupation by any means, will be exempted from the attacks of the mujahedeen," said the statement, which could not be immediately authenticated.

The statement singled out three radical Shiite clerics and their followers who were exempted from the declaration of war: al-Sadr and Baghdad-based anti-U.S. religious leaders Jawad al-Khalisi and Ahmed al-Hassani al-Baghdadi.

The Karbala festival marked the 868 A.D. birth of Imam Mohammed al-Mahdi, whose unexplained disappearance in the 9th century has led Shiites to believe he will reappear on doomsday to lead believers to a just Islamic state.

Stomping on pictures of al-Zarqawi that were thrown onto the streets of Karbala, many pilgrims chanted praise for their own religious leaders while others rejected the Al Qaeda threat.

"Even if there were a million al-Zarqawis, we would come to visit the Imam Hussein (shrine)," a Shiite holy place named for the grandson of the prophet Muhammed.

Police said they seized weapons caches near the city, and arrested three "non-Iraqi Arabs" and a man armed with several hand grenades as he walked in the procession of pilgrims.

Authorities were keen to avoid a repeat of past tragedies during mass religious gatherings in Karbala and other holy places where suicide bombers have mounted coordinated attacks that left hundreds dead.

On Aug. 21, nearly 1,000 pilgrims were killed in a stampede on a Baghdad bridge that began when rumors spread through the crowd that a suicide bomber was among the faithful.