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 (search).

The new violence came as the Pentagon announced it was ordering 1,500 paratroopers to Iraq to provide security in advance of two upcoming national votes. Two infantry battalions from the 82nd Airborne Division (search) will deploy to Iraq before the scheduled Oct. 15 referendum on the proposed constitution, and remain through the December national elections.

The confrontation in the south — 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.

The trouble in the south began when supporters of radical cleric Muqtada al-Sadr (search) tried to reopen his office in Najaf, which was closed after the end of fighting there last year.

When Shiites opposed to al-Sadr tried to block the move, fighting between the two groups broke out, killing at least four people.

In wake of the Najaf incident, clashes between al-Sadr's forces and a rival Shiite group broke out Wednesday night in at least three southern cities, and Prime Minister Ibrahim al-Jaafari appeared on national television to appeal for calm.

"The battle should not be between the people of Iraq but against the enemies of Iraq. The language of guns has gone forever," said al-Jaafari, a Shiite. "I call upon all the sons of Najaf to know the dangers that are threatening the country and close the door for those trying to take advantage of the situation."

Twenty-one lawmakers and three senior government officials allied with al-Sadr refused to carry out duties indefinitely to protest the attack in Najaf. The officials were said to be Transport Minister Salam al-Maliki, Health Minister Abdel Mutalib Mohammed and the minister of state for civil affairs, Alaa Habib.

Unless the crisis within the majority Shiite political establishment can be resolved quickly, it would seriously complicate efforts to convene parliament soon to vote on a new constitution. Sunni Arabs are insisting on consensus on the draft, which contains several provisions to which they object. Most Shiite politicians have agreed to the draft.

Without al-Sadr's representatives present, it would be difficult for supporters of the draft to argue that it had won the acceptance of leaders of all Iraqi communities. The dispute also raises fears of Shiite-Shiite violence at a time when U.S. and Iraqi leaders are already facing a virulent Sunni-led insurgency.

Al-Sadr's followers blamed the rival Supreme Council for the Islamic Revolution in Iraq, or SCIRI, for the Najaf attack. SCIRI and its Badr Brigade militia both denied involvement and condemned the violence.

But as word of the Najaf incident spread, clashes broke out in major cities of the Shiite south, including the country's second largest city, Basra. Police said eight mortar shells were fired at SCIRI's headquarters and clashes broke out in the city center between the two Shiite groups.

Al-Sadr's office was surrounded by his Mahdi Army militia, which sent out a call to its members throughout the province to come and defend the building.

Clashes were also reported in Nasiriyah and Amarah, where mortar shells were also fired at SCIRI offices. There were no reports of casualties.

Interior Minister Bayan Jabr, a member of SCIRI, told Iraqiya television that "we will not allow anyone to violate the law" and that a commando brigade was dispatched to Najaf. A curfew was imposed from 11 p.m.

Jabr said such acts will delay a pullout by U.S. forces from the city.