Six of Hakim's seven brothers were killed during Saddam Hussein's rule
Abdul Aziz al-Hakim, the leader of the Shia Supreme Iraqi Islamic Council (SIIC), had a long history of opposition to the rule of Saddam Hussein, and became one of Iraq's most powerful figures after his downfall.
Hakim lived in exile in Iran for more than two decades before returning home in April 2003, one month after the US-led invasion.
Following the assassination in August of that year of his brother, Ayatollah Muhammad Baqr al-Hakim, he became head of SIIC's precursor, the Supreme Council for the Islamic Revolution in Iraq (Sciri).
Attacked for co-operating with the Coalition authorities and accused by others of being an Iranian agent, he nevertheless helped lead the Shia-dominated United Iraqi Alliance (UIA) to victory in the January and December 2005 parliamentary elections.
Despite Sciri's political power and popular support, Hakim stepped back from the limelight and allowed other senior members to come to the fore. However, few doubted the fact that he was still in control.
In 2007, with support for the UIA-led government and Sciri dwindling amid widespread sectarian violence and claims of rampant corruption, he was diagnosed with lung cancer.
After a long battle, during which he was treated by doctors in Iran and the West, Hakim died in a hospital in Tehran on Wednesday.
Born in 1950, Abdul Aziz al-Hakim came from a prominent family from the holy city of Najaf. His father, Grand Ayatollah Muhsen al-Hakim, was the most widely-followed marja, or source of emulation, of the Shia world until his death in 1970.
Sciri was accused of allowing its military wing operate death squads
After his death, Hakim and the family continued the ayatollah's support of Shia Islamist opposition groups, campaigning against the secular, Sunni-dominated Baathist regime.
The Iraqi government responded by suppressing the Shia community, their leaders and traditions. Six of Hakim's seven brothers were killed, while more than 50 members of his extended family were executed.
Shortly before the outbreak of the Iran-Iraq War in 1980, Hakim and his elder brother, Muhammad, left Najaf for Iran.
In 1982, the two brothers helped found Sciri, a coalition linking a number of Iraqi Shia opposition organisations that called for the establishment of an Islamic order in Iraq.
Muhammad Baqr soon became the group's chairman, while Abdul Aziz occupied the important posts of deputy chairman and commander of its military wing, the Badr Brigade.
Funded and trained by Iran, the Badr Brigade fought alongside Iranian troops against the Iraqi army in Kurdistan during the Iran-Iraq War. It was believed to have 20,000 fighters prior to the 2003 invasion.
In 1991, following the end of the Gulf War, thousands of Badr fighters crossed over the border into Iraq to support the Shia rebellion in the South, but it was swiftly suppressed by the state.
Hakim voted in January's provinical elections despite his ill-health
In 2002, Hakim led a Sciri delegation to Washington for meetings with the US government and other opposition groups. Despite initially rejecting a "foreign invasion", the group eventually welcomed US help.
Hakim returned to Iraq shortly after US forces entered central Baghdad, amid rumours that his brother would hand over the leadership of Sciri to him to concentrate on spiritual leadership.
Muhammad Baqr returned triumphantly to Basra a month later. Despite calling for independence from foreign powers, he supported the appointment of Abdul Aziz to the Governing Council of Iraq by the US-led Coalition Provisional Authority that July.
Later the following month, he was killed when a car-bomb exploded outside the shrine of Imam Ali in Najaf. Abul Aziz was elected Sciri chairman five days later and vowed to continue his work.
Shortly after his election, coalition officials banned party militias. In response, Hakim announced that the Badr Brigade would be renamed the "Badr Organisation" and would play a part in building the new Iraq. But it would also still have an important security role, he said.
Hakim's pledge to co-operate with Coalition authorities placed him in direct confrontation with newer, more radical Shia clerics who had stayed in Iraq under Saddam Hussein, such as Moqtada Sadr.
Sadr accused Sciri of attempting to benefit from the peace deal that saw his Mehdi Army leave Najaf after a bloody battle with US troops, and warned them to "beware that they are not sucked into America's plot to incite fighting among Shia".
Hakim and Sciri were collaborators in the eyes of Sunni militants too. Al-Qaeda in Iraq claimed responsibility for a bomb attack on Sciri's headquarters in 2004 which Hakim survived but left 13 dead.
Hakim's son, Ammar, is expected to take over the leadership of SIIC
Following the January 2005 election, several Sciri leaders were given key roles in the new Iraqi cabinet - Adel Abdul Mahdi became vice-president, while Bayan Baqir Solagh was named interior minister.
Mr Solagh was heavily criticised for the human rights abuses committed by interior ministry personnel during his term, and accused by many Sunni Arabs of allowing Shia militias, including the Badr Organisation, to operate death squads within the security forces.
The UIA maintained its dominance in the December 2005 election despite the outrage, but this time Sciri was forced to accept a compromise candidate from the rival Dawa Party, Nouri Maliki, for the post of prime minister.
In the three years since coming to power, Mr Maliki has sought to reduce the presence of Shia militiamen in the security forces and limit Sciri's power, while at the same time draw Sunni Arabs back into the political process. This, combined with the US troop surge, has seen security dramatically improve in the past year.
The changing situation and its questionable government record has also resulted in diminished support for Sciri - which dropped its revolutionary tag and became the Supreme Iraqi Islamic Council in 2007 - but it nevertheless remains one of the most popular, powerful and well-organised political parties in Iraq.
Hakim's son, Ammar, is expected to take over the leadership of SIIC, in the hope of ensuring a smooth succession ahead of the next national elections in January, for which the party will be part of a Shia-led coalition excluding Mr Maliki and the Dawa Party.