After the toppling of Saddam Hussein his militia fought fierce battles with American troops and he was identified by the Pentagon in 2006 as the biggest threat to stability in Iraq.

But after years on the sidelines, Sadr linked up with secularists with a promise to battle corruption and now appears to hold the keys to Baghdad.

The nationalist cleric's Marching Towards Reform alliance came out on top in Iraq's parliamentary elections earlier this month.

"Sadr -- often dubbed a firebrand cleric -- has come a long way from the days in 2003 when he was an outcast and a hunted man," said Nabeel Khoury, from the Atlantic Council think tank.

The election success follows three years of weekly protests, with Sadrists rallying alongside communists to call for an overhaul of the political class.

While Sadr has ruled himself out of becoming prime minister, he should become kingmaker and aims to form a technocratic government from a dozen parties.

Such a victory over internationally favoured prime minister Haider al-Abadi came as a surprise, but Sadr is well-known to Iraqis and US forces alike.

- Militia army -

His rise to political power has been aided by the reputations of two famed relatives -- including his father, Grand Ayatollah Mohammed Mohammed Sadiq al-Sadr -- killed during Saddam Hussein's rule.

With a grey bushy beard and wearing the black turban of a "sayyid" or descendant of the Prophet Mohammed, Sadr gained widespread popularity in the months after the 2003 invasion of Iraq.

His Mahdi Army -- estimated to have had up to 60,000 members -- was once the most active and feared armed Shiite group in the country, and was blamed by Washington for death-squad killings of thousands of Sunnis.

But in August 2008, Sadr suspended the activities of the Mahdi Army after major US and Iraqi assaults on its strongholds in Baghdad and southern Iraq.

Following the ceasefire, US military commanders said Sadr's action had been instrumental in helping bring about a significant decrease in the levels of violence across Iraq.

He nonetheless continued his vocal opposition to the US military presence in the country.

- Holding onto the street -

Despite repeatedly bowing out of politics, reportedly pursuing religious studies in the Iranian holy city of Qom, he remained able to pull powerful political strings.

After throwing his weight behind Shiite politician Nuri al-Maliki in 2006, ensuring he became prime minister, Sadr then ordered his followers to pull out of the cabinet the next year, almost bringing down the government.

Sadr's bloc contested the 2010 legislative election in an alliance with the Supreme Iraqi Islamic Council, another Shiite group with links to Iran.

The inconclusive polls saw Maliki gain a second term as premier -- during which he was deemed a "dictator" by Sadr who called for his resignation.

More recently the former commander renewed his militia to defend Iraqi religious sites in 2014, after the Islamic State group overran large areas north and west of Baghdad.

Whether on a military, clerical or political path, Sadr has been careful to maintain the power base which has now given him an unrivalled level of political influence.

"Because he holds onto the street, he upsets numerous parties," said Iraqi political expert Essam al-Fili.

For the analyst, forming a coalition government with Sadr would result in "the political situation entering a phase of instability".