Clashes between rival Shiite militia groups occured in Baghdad's Sadr City neighborhood on Friday, when one militia launched an attack on the other's headquarters.
The violence between the Mahdi Army loyal to radical Shiite cleric Muqtada al-Sadr and the Badr Brigades of the Supreme Council for the Islamic Revolution in Iraq began earlier Friday when Iraqi police loyal to the Badr Brigades prevented a Sadr aide from entering the southern Shiite holy city of Najaf.
A brief fight broke out between the two sides, injuring four people, hospital officials said. Police in Najaf, 160 kilometers (100 miles) south of Baghdad, imposed a curfew beginning 8 p.m. Friday.
Two mortar shells later hit a SCIRI building on the edge of Sadr City, injuring two guards, witnesses said.
SCIRI gunmen - apparently blaming the Mahdi Army for the attack - began blocking roads near their damaged building, but quickly withdrew, police said.
Mahdi army fighters then attacked the empty building with rockets and gunfire, setting part of it on fire, police said.