In a statement read out to thousands of supporters gathered on one of the main access roads to the capital's Green Zone government and diplomatic compound, the Framework said it would hold an "indefinite sit-in" to push for the swift formation of a new government to end months of deadlock between the rival Shiite blocs.

An AFP correspondent saw protesters begin assembling large metal tent frames in readiness to camp out.

Loyalists of Moqtada Sadr, a revered Shiite cleric who once led a militia against US and Iraqi government forces, have been demanding fresh elections after the Coordination Framework named a candidate for the premiership, a post they believe should be theirs.

The Coordination Framework brings together the party of ex-prime minister Nuri al-Maliki, a longtime Sadr foe, and the Hashed al-Shaabi, a pro-Iran former paramilitary network now integrated into the security forces.