Clashes erupted between rival Shia groups across the Shia-dominated south of Iraq yesterday 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 Shia leader who led two uprisings against US forces last year.
Trouble in the south began when supporters of radical cleric Muqtada al-Sadr tried to reopen his office in the Shia holy city of Najaf, which was closed after the end of fighting there last year.
When Shias 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 Shia 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, said he was sending 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 in protest at the Najaf attack.
The violence in the south 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.