A firebrand with millions of followers and in command of paramilitary groups, Sadr is a crucial player in Iraqi politics who has often protested against the influence of both the United States and Iran.

Sadr had said in mid-July that he would not participate in the October 10 parliamentary election and would withdraw support from "anyone who claims they belong to us in this current and upcoming government."

He reversed that position on Friday, saying he had received pledges from "certain" political leaders to reform the country and "put an end to corruption".

Taking part in the elections is "now acceptable", he said during a televised address, flanked by dozens of officials from his Sadrist movement.

Sadr, whose political manoeuvres have at times puzzled observers, in February had said he backed early elections overseen by the UN.

Militias loyal to Sadr fought the US-led occupation of Iraq and he retains a devoted following among the country's majority Shiite population, including in the poor Baghdad district of Sadr City.

The son of a revered religious figure, Sadr wears a black turban that signals descent from the family of the Prophet Mohammed.

The parliamentary vote is set to be held under a new electoral law that reduces the size of constituencies and eliminates list-based voting in favour of votes for individual candidates.

Prime Minister Mustafa al-Kadhemi, who came to power in May last year after months of unprecedented mass protests against a ruling class seen as corrupt, inept and subordinate to Tehran, had called the early vote in response to demands by pro-democracy activists.

Sadr's supporters have been expected to make major gains under the new electoral system.

His Saeroon bloc is currently the largest in parliament, with 54 out of 329 seats.

Plagued by endemic corruption, poor services, dilapidated infrastructure and unemployment, Iraq is facing a deep financial crisis compounded by lower oil prices and the Covid-19 pandemic.

Sadr has appeared under pressure in recent weeks, with pro-Iran groups and individuals attacking him on social media and accusing him of responsibility for Iraq's recent woes, including electricity shortages and two deadly hospital fires.