The son of the late leader of Iraq's largest Shiite Muslim political party took the reins of the Supreme Islamic Iraqi Council, a party official said.
The choice of Ammar Hakim to succeed his father, Abdelaziz Hakim, was widely expected. But there have been questions whether the relatively inexperienced son can hold the group together during a politically sensitive time.
The late Hakim was a symbol to the Shiite political majority of the victory over Saddam Hussein's Sunni-led regime, but the party did poorly in many parts of the Shiite south in local elections this year.
Party leaders have said they would stand behind Hakim, who took charge of the party's affairs when his father withdrew after being diagnosed with lung cancer in May 2007.
The elder Hakim died last week in neighboring Iran, where he was undergoing treatment.