Sadr, a longtime political and religious force in the oil-rich but war-scarred country, has for months been in a political standoff with a rival Shiite alliance backed by Iran.

Worshippers converged on a vast square inside the normally secure Green Zone, home to government and diplomatic buildings, including the parliament which his followers began occupying on July 30.

"Yes, yes to reform! Sadr's followers chanted during the prayers.

"No, no to corruption."

After the prayers, hundreds returned to the vicinity of parliament, whose air conditioned halls and marble floors they had occupied since last Saturday.

Finding the doors locked and the building empty, they continued their sit-in among the gardens of the complex, according to a request from their movement. Mixed signals emerged Tuesday from the Sadr camp for the inside of parliament to be cleared.

Sadr's mass prayer rally follows his demand for early elections -- a possibility that the rival bloc says it is conditionally open to, despite the last national polls only taking place about 10 months ago.

Since then, post-election negotiations between Sadr's bloc -- the largest in parliament -- and other factions have failed to produce a new government, prime minister and president.

- 'Prisoner of the corrupt' -

The political tensions come as Iraq remains beset by rampant corruption, crumbling infrastructure and unemployment.

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

Supporters of Sadr, however, are ready to follow him almost blindly and view him as a champion of the anti-corruption fight.

Speaking at a lectern, the imam who led the prayer endorsed Sadr's call for early elections.

"Iraq is a prisoner of the corrupt," the imam said, denouncing "the scandalous deterioration of public services, health and education".

Sheikh Ali al-Atabi, 38, joined the throng to support Sadr. Calling people to Friday prayers is "part of his repertoire" when he "wants to use the people for something," Atabi explained.

A similar prayer call and pressure tactic from Sadr in mid-July drew hundreds of thousands of Muslim worshippers to Sadr City, a Baghdad district named after his assassinated father.

Qassem Abu Mustafa, 40, described the latest gathering as "a thorn" jabbing "the enemy to demand legislative elections and reforms".

The faithful, mostly men but with some women, used umbrellas to protect themselves from Baghdad's 42 degrees Celsius (108 Fahrenheit) heat.

Some waved Iraqi flags and carried portraits of their leader.

"Whatever Mr Sadr's opinion, we are with him," said Abu Mustafa, a civil servant.

- Devoted following -

Sadr's bloc emerged from the October elections as parliament's biggest, but still far short of a majority.

In June, his 73 lawmakers quit in a bid to break the logjam. That led to a rival Shiite bloc, the pro-Iran Coordination Framework, becoming the largest in the legislature.

The Coordination Framework's nomination of former cabinet minister Mohammed Shia al-Sudani as prime minister angered the Sadr bloc and triggered the occupation of parliament by his supporters.

With armed groups linked to the various political factions in Iraq, the United Nations has warned that tensions could escalate.

On Wednesday, Sadr called for the dissolution of parliament and new polls. The Coordination Framework late Thursday said they were open to that idea, signalling a potential deescalation.

But "a national consensus on the question and providing a safe environment" were prerequisites for such polls, it said, also suggesting the occupation of parliament must end.

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

Outgoing parliamentary speaker Mohammed al-Halbussi, a member of the minority Sunni community, on Twitter expressed support for new elections.

He said it is "impossible to ignore the will of the masses".