Mr Sadr, 48, has been a dominant figure in Iraqi public and political life for the past two decades. His Mehdi Army emerged as one of the most powerful militias which fought US and allied Iraqi government forces in the aftermath of the invasion which toppled former ruler Saddam Hussein.

He later rebranded it as the Peace Brigades, and it remains one of the biggest militias which now form part of the Iraqi armed forces.

Although the Mehdi Army had links to Iran, Mr Sadr had latterly distanced himself from Iraq's Shia neighbour and repositioned himself as a nationalist wanting to end US and Iranian influence over Iraq's internal affairs.

The rival Shia political bloc, the Coordination Framework, with which Mr Sadr's bloc has been at loggerheads, mainly includes Iran-backed parties, according to BBC.

He is one of a few figures who could quickly mobilise hundreds of thousands of supporters onto the streets, and draw them down again. Hundreds have been camped outside parliament since storming it twice in July and August in protest at the deadlock.