Baghdad, Iraq ? Clashes erupted between rival Shiite groups across the Shiite-dominated south Wednesday, threatening Iraq with yet another crisis at a time when politicians are struggling to end a constitutional stalemate with Sunni Arabs.

The confrontation in at least five southern cities – involving a radical Shiite leader who led two uprisings against U.S. forces last year – followed the boldest assault by Sunni insurgents in weeks in the capital.

Dozens of insurgents wearing black uniforms and masks attacked Iraqi police in western Baghdad with multiple car bombs and small-arms fire that killed at least 13 people and wounded 43, police said.

Trouble in the south began when supporters of radical cleric Muqtada al-Sadr tried to reopen his office in the Shiite holy city of Najaf, which was closed after the end of fighting there last year.

When Shiites opposed to al-Sadr tried to block the move, fights broke out. Four people were killed, 20 were injured and al-Sadr’s office was set on fire, police said.

That enraged al-Sadr’s followers, who blamed the country’s biggest Shiite party, the Supreme Council for the Islamic Revolution in Iraq, or SCIRI.

The party, which controls key posts in the national government, quickly denied responsibility and condemned the attack. Interior Minister Bayan Jabr, a member of SCIRI, told Iraqiya television he was dispatching a commando brigade to Najaf to restore order. A curfew was imposed from 11 p.m.

Despite the government’s move, 21 pro-al-Sadr members of parliament and three senior Cabinet officials announced they would refuse to perform their duties indefinitely to protest the Najaf attack.

Municipal officials loyal to al-Sadr in several southern cities issued similar declarations.

As word of the Najaf attack spread, clashes broke out between the two Shiite rival groups across central and southern Iraq. The violence extended to the country’s second largest city, Basra, where several hardline Shiite groups are competing for influence.

Fighting was reported in at least six Basra neighborhoods as al-Sadr’s followers attacked SCIRI offices and the headquarters of SCIRI’s Badr Brigade militia, setting it ablaze, police said. Al-Sadr’s headquarters in Basra was attacked with rocket-propelled grenades and small arms fire, according to police.

In Amarah, eight mortar shells were fired at the SCIRI office, and a dozen pro-al-Sadr officials announced they were also suspending work. Gunmen from al-Sadr’s militia roamed the streets. Clashes were also reported in Kut, where a SCIRI-owned building was torched, and in Nasiriyah.

Pedestrians approach a burning police vehicle in the western Baghdad neighborhood of Khadra after a firefight between insurgents and Iraqi police. Insurgents attacked Iraqi police patrols in western Baghdad on Wednesday with multiple car bombs and small-arms fire, killing at least 13 people and wounding 43, police said. Elsewhere in Iraq, a wave of attacks among Shiite groups further unsettled the country as work to seek approval for a constitution continued.

Faced with yet another crisis, Prime Minster Ibrahim al-Jaafari, also a Shiite, appeared on Iraqiya television shortly before midnight to call for restraint.

“The battle should not be between the people of Iraq but against the enemies of Iraq,” al-Jaafari said, using language reserved for the insurgents. “The language of guns has gone forever.”

Although Wednesday’s clashes could end as quickly as they began, they were ominous, coming at a time when Iraq faces an increasingly bold insurgency and a difficult constitutional process that has exacerbated rather than calmed religious and ethnic tensions.

Late Monday, parliament delayed a vote on the new constitution after Sunni Arab negotiators rejected a draft accepted by Shiite and Kurdish officials. On Wednesday, President Jalal Talabani, a Kurd, said stability cannot be achieved without consensus among Iraq’s Shiites, Sunnis Arabs and Kurds. Representatives of the communities were to meet today.

U.S. and Iraqi officials hope a new constitution will help curb violence by luring disaffected Sunnis away from the Sunni-dominated insurgency. That would allow the United States and its partners to begin bringing their troops home starting next year.