Shortly after he made his surprise declaration, Sadr followers "entered the Republican Palace", a government building inside Baghdad's fortified Green Zone that also houses diplomatic missions, a security source said.

Inside the palace, protesters lounged in armchairs in a meeting room, with others waving Iraqi flags and taking photographs of themselves -- while others cooled off in a pool in the garden, an AFP photographer said.

As several thousand other loyalists -- many shouting "Moqtada, Moqtada" -- headed towards the Green Zone, an AFP journalist reported, the army announced "a full curfew in the capital Baghdad" starting from 3:30 pm (1230 GMT).

Since legislative elections in October last year, political deadlock has left the country without a new government, prime minister or president, due to disagreement between factions over forming a coalition.

- 'Definitive retirement -

Sadr -- a grey-bearded preacher with millions of devoted followers, who once led a militia against American and Iraqi government forces following the toppling of dictator Saddam Hussein -- had announced earlier on Monday on Twitter he was stepping back from politics.

"I've decided not to meddle in political affairs. I therefore announce now my definitive retirement," said Sadr, a longtime player in the war-torn country's political scene, though he himself has never directly been in government.

He added that "all the institutions" linked to his Sadrist movement will be closed, except the mausoleum of his father, assassinated in 1999, and other heritage facilities.

His latest statement came two days after he said "all parties" including his own should give up government positions in order to help resolve the months-long political crisis.

His bloc emerged from last year's election as the biggest, with 73 seats, but short of a majority. In June, his lawmakers quit in a bid to break the logjam, which led to a rival Shiite bloc, the pro-Iran Coordination Framework, becoming the largest in the legislature.

Since then, Sadr has engaged in other pressure tactics, including a mass prayer by tens of thousands of his followers on August 5.

- Millions of followers -

His supporters have been calling for parliament to be dissolved and for new elections, but on Saturday he said it is "more important" that "all parties and figures who have been part of the political process" since the 2003 US-led invasion "no longer participate".

"That includes the Sadrist movement," he said, adding that he was willing to sign an agreement to that effect "within 72 hours".

Over the years, the chameleon-like Sadr has taken various positions and then reversed them.

Sadr's supporters have for weeks been staging a sit-in outside Iraq's parliament, after initially storming the legislature's interior on July 30, to press their demands.

They were angered after the Coordination Framework nominated a candidate they saw as unacceptable for prime minister.

The Framework wants a new head of government to be appointed before any new polls are held.

Caretaker Prime Minister Mustafa al-Kadhemi earlier this month convened crisis talks with party leaders, but the Sadrists boycotted.

Iraqis say the political infighting has nothing to do with their day-to-day struggles.

Iraq has been ravaged by decades of conflict and endemic corruption. Oil-rich but blighted by ailing infrastructure, unemployment, power cuts and crumbling public services, Iraq now also faces water shortages as drought ravages swathes of the country.

As a result of past deals, the Sadrists have representatives at the highest levels of government ministries and have been accused by their opponents of being as corrupt as other political forces.

But supporters of Sadr view him as a champion of the anti-corruption fight.