Hakim was diagnosed with lung cancer in 2007 and had chemotherapy
Abdul Aziz al-Hakim, leader of one of the most powerful Shia Muslim parties in Iraq, has died, his aides say.
Hakim had been suffering from cancer and had been receiving treatment in hospital in the Iranian capital Tehran.
He did not hold any government post in Iraq's elected Shia-led government since the overthrow of Saddam Hussein, but was an important power broker.
He took control of the Sciri party (later SIIC) after his brother was assassinated in Najaf in 2003.
The cleric, who was in his late 50s, was diagnosed with lung cancer during a trip to the US for tests in May 2007. He chose to receive chemotherapy treatment in Tehran.
The party has several senior cabinet members, and its militia - the Badr Brigade - has at times wielded considerable influence in Iraq's security establishment.
Since falling ill, Hakim had cut back his political involvement and his son Ammar gained prominence. He is expected to take over leadership of the party.
ABDUL AZIZ AL-HAKIM
Born circa 1950, died 26 August 2009
Leader of Islamist Shia party Sciri, later SIIC, since 2003
Back by Tehran, but maintaining close ties to its arch-rival Washington
Lost six of his seven brothers and 50 extended family members in resistance to Saddam Hussein
"He died a few minutes ago after battling cancer for 28 months," his son Mohsen announced in Tehran at about midday on Wednesday. He and his brother Ammar were at their father's bedside as he died, he added.
As heir to the leadership of one of the main anti-Saddam Hussein factions in Iraq, Abdul Aziz Hakim managed to keep good ties with both the American authorities and Iran, which strongly backed his group.
His brother and predecessor as party leader was the charismatic Ayatollah Muhammad Baqr al-Hakim, who was killed along with about 100 supporters in a massive car bombing in the city of Najaf in August 2003.
The family is revered among Iraq's largest religious community, the Shia, for its tradition of scholarship and its bouts of resistance against Saddam Hussein in its southern Iraqi stronghold.
However, the quietly-spoken Hakim was distrusted by many Sunnis who saw him as too Iranian-orientated and sectarian in his political philosophy.
Some time after the overthrow of Saddam Hussein, the party changed its name from Sciri - the Supreme Council for the Islamic Revolution in Iraq - to the Supreme Islamic Iraqi Council.
SIIC has been part of Iraq's ruling Shia alliance, the United Iraqi Alliance, led by the Islamic Dawa party of Prime Minister Nouri Maliki.
However, ahead of national elections in January, the SIIC announced last week that it would campaign from within a new Shia Muslim bloc.