A string of mass protests culminating in an ongoing sit-in at the gates of Baghdad's Green Zone have thrust the mercurial Shiite cleric Moqtada Sadr back on to centre stage.

The scion of an influential clerical family from the holy city of Najaf, he first made a name for himself at the age of 30 as a vociferous anti-American cleric who raised a rebellion.

His influence ebbed after the 2011 US pullout but he retained strong support among the lower classes and is now casting himself as the champion of the fight against graft.

"This is your time to root out corruption and the corrupt," he said earlier this month in a call to his supporters to march on the fortified Green Zone and set up protest camps.

Thousands of them defied a government ban on Friday to heed their leader's call and set up tents at the main entrances of the vast restricted area in central Baghdad which houses key state institutions as well as foreign embassies.

Sadr says the goal of the protests is the formation of a cabinet of technocrats to replace party-affiliated politicians he says have perpetuated a system based on nepotism and patronage.

Prime Minister Haider al-Abadi himself made the proposal but he is being undermined by parties -- including his own -- whose barons are reluctant to relinquish their positions and attendant privileges.

While ostensibly declaring his support for Abadi's proposed reforms, Sadr's decision to take to the street leaves the prime minister even closer to the brink.

The sit-in and the huge security deployment around it have paralysed central Baghdad and Sadr has given Abadi an ultimatum expiring in a week to present names for a new cabinet.

"This is a serious escalation," said Ahmed Ali, of the Institute of Regional and International Studies at the American University of Iraq.

"Sadr started this thing and will not go silent now, he wants to go all the way," said Issam al-Faily, professor of political science at Baghdad's Mustansiriya University.

- Unpredictable -

Since he took over the premiership in 2014, and despite the backing of the country's top Shiite cleric, Grand Ayatollah Ali al-Sistani, Abadi has ruled with very tenuous support from his own government.

Now any reform that does go through would risk looking like a victory for Sadr and create more unease among the country's other, rival Shiite leaders.

"The Sadrists are attempting to reinsert themselves forcefully into the Iraqi Shiite political sphere which is getting more contentious by the moment," said Ahmed Ali.

The top players in the Shiite-majority country, many of them aligned with Iran, are fretting at the resurgence of the populist cleric, who was once a Tehran client but has since reinvented himself as a nationalist.

While his popularity had appeared to recede in recent years, the 42-year-old cleric can still mobilise large crowds like few others in Iraq.

"Sadr began this as a new effort to co-opt the anti-corruption protest movement after a different effort failed last fall," said Kirk Sowell, publisher of the Inside Iraqi Politics newsletter.

Several stalwarts of the Sadr movement are in the government and some are widely seen as the least competent and more corrupt but the cleric has tried to deflect any criticism by distancing himself from his own ministers.

"The solution should come from all parties. Once they are satisfied they can wield their influence through parliament, they should accept the formation of an independent government," said Faily.

Possibly the most predictable feature of Sadr is his unpredictability.

"Impossible to know where this is going," said Sowell. "How he actually accomplishes anything meaningful, I don't know. It may help him in becoming the popular leader he wants to be."

In a statement on Saturday, Sadr said he did not want to complain -- but essentially did -- about "the lack of coverage by Iraqi, Arab and international channels of the most important event in Iraq, the peaceful national sit-in."