Moqtada Sadr has been a powerful figure in Iraq since the fall of Saddam Hussein.
Although the situation has changed in the country since the radical Shia cleric went into self-imposed exile in Iran in 2007, he appears to have has lost none of his influence and has maintained his wide support among many of Iraq's impoverished Shia Muslims.
At times he has called for a national rebellion against foreign troops and sent out his Mehdi Army militiamen to confront the "invaders" and Iraqi security forces.
At others he has appeared more compromising, seeking for himself a political role within the new Iraq and helping form the national unity government in December 2010.
He returned to Iraq on 5 January 2011. Weeks before the withdrawal of US troops from the country, as negotiations were ongoing between Baghdad and Washington over a possible extension of their mission, he threatened to reactivate the Mehdi Army in case an extension is agreed.
The youngest son of Grand Ayatollah Muhammad Sadiq Sadr - who was assassinated in 1999, reportedly by Iraqi agents - Moqtada Sadr was virtually unknown outside Iraq before the March 2003 invasion.
But the collapse of Baathist rule revealed his power base - a network of Shia charitable institutions founded by his father.
In the first weeks following the US-led invasion, Moqtada Sadr's followers patrolled the streets of Baghdad's Shia suburbs, distributing food, providing healthcare and taking on many of the functions of local government.
They also changed the name of the Saddam City area to Sadr City.
Moqtada Sadr also continued his father's practice of holding Friday prayers to project his voice to a wider audience.
The practice undermined the traditional system of seniority in Iraqi Shia politics and contributed to the development of rivalries with two of Iraq's Grand Ayatollahs, Kazim al-Hairi and Ali Sistani.
Moqtada Sadr drew attention to their links with Iran, whose influence on Iraq's political and religious life his followers resented.
He also called on Shia spiritual leaders to play an active role in shaping Iraq's political future, something most avoided.
Moqtada Sadr also used his Friday sermons to express vocal opposition to the US-led occupation and the Iraqi Governing Council (IGC).
In June 2003, he established a militia group, the Mehdi Army, pledging to protect the Shia religious authorities in the holy city of Najaf.
He also set up a weekly newspaper, al-Hawzah, which the Coalition Provisional Authority (CPA) banned in March 2004 for inciting anti-US violence. The move caused fighting to break out between the Mehdi Army and US-led coalition forces in Najaf, Sadr City and Basra.
The following month, the US said an Iraqi judge had issued an arrest warrant for Moqtada Sadr in connection with the murder of the moderate Shia leader, Abdul Majid al-Khoei, in April 2003. Moqtada Sadr strongly denied any role.
During the negotiations for a truce, the Americans also reportedly agreed to lay aside the warrant for Moqtada Sadr.
The fierce clashes continued in Sadr City, however, and only ended in October after the Mehdi Army had sustained heavy losses.
Though costly, the violence cemented Moqtada Sadr's standing as a force to be reckoned with in Iraq.
He became a symbol of resistance to foreign occupation - a counterpoint to established Shia groups such as the Supreme Council for the Islamic Revolution in Iraq (Sciri) and the Daawa Party.
Despite this, Moqtada Sadr chose to join his rivals' coalition for the December 2005 elections - the United Iraqi Alliance (UIA).
The alliance had easily won Iraq's first post-invasion election the previous January, and with the Sadr Bloc on board again came out on top.
In the months of government negotiations that followed, Moqtada Sadr used his influence to push for the appointment of Nouri Maliki, then Daawa's deputy leader, as prime minister. In return, his supporters got powerful positions in the cabinet.
At the same time, extremist Sunni Islamist militant groups - increasingly supported by Iraq's marginalised Sunni Arab minority - had begun to target the Shia community, not just foreign troops.
Insurgents attacked Shia Islam's most important shrines and killed many Shia politicians, clerics, soldiers, police and civilians.
As the sectarian violence worsened, the Mehdi Army was increasingly accused of carrying out reprisal attacks against Sunni Arabs.
In 2006 and 2007, thousands of people were killed as the sectarian conflict raged. The Iraqi security forces seemed unable to stop the violence, though many blamed this on the infiltration of the interior and defence ministries by the Mehdi Army and other Shia militias.
One Pentagon report described the Mehdi Army as the greatest threat to Iraq's security - even more so than al-Qaeda in Iraq. Iran was accused of arming it with sophisticated bombs used in attacks on coalition forces.
Then in early 2007, after US President George W Bush ordered a troop "surge" in Iraq, it was reported that Moqtada Sadr had left for Iran and told his supporters
In August 2007, heavy fighting broke out between the Mehdi Army and Sciri's Badr Brigade in Karbala, leaving many dead.
The internecine fighting was condemned by many Shia, and Moqtada Sadr was forced to declare a ceasefire.
In March 2008, Mr Maliki ordered a major offensive against the militia in the southern city.
At first, the Mehdi Army seemed to have fended off the government's attempts to gain control of Basra. But within weeks, it had accepted a truce negotiated by Iran, and the Iraqi army consolidated its hold.
US and Iraqi forces also moved into Sadr City, sparking fierce clashes but also eventually emerging victorious.
In August 2008, Moqtada Sadr ordered a halt to armed operations. He declared that the Mehdi Army would be transformed into a cultural and social organisation, although it would retain a special unit of fighters who would continue armed resistance against occupying forces.
He meanwhile devoted his time to theological studies in the Iranian holy city of Qom, in the hope of eventually becoming an ayatollah.
Analysts say the title would grant him religious legitimacy and allow him to mount a more serious challenge to the conservative clerical establishment in Iraq.
At the same time, he built on the gains of the Sadr Bloc in the 2005 elections to increase his political influence. His supporters performed strongly in the 2009 local elections and made gains in the March 2010 parliamentary polls as the Iraqi National Alliance (INA), ending up with 40 seats.
The result made Moqtada Sadr the kingmaker in the new parliament. He toyed initially with backing Mr Maliki's rival for the premiership, but in June agreed to a merger between the INA and the prime minister's State of Law coalition.
Then in October, he was finally persuaded by Iran to drop his objection to Mr Maliki's reappointment in return for eight posts in the cabinet.
Secure in his standing, Moqtada Sadr returned from Iran in January to scenes of jubilation.