The blast occurred outside the offices of the Supreme Council for the Islamic Revolution in Iraq (SCIRI) in Baghdad. However, Shia leader Abdel Aziz al-Hakim, believed to be the target of the attack, was at his home at the time.
SCIRI, a religious party formed by exiled Iraqi Shias in Iran during the Saddam Hussein regime, is expected to play a major role in Iraq's national elections scheduled for 30 January.
At least three members of the Iraqi Hezbollah party were among those killed in the explosion, which also damaged 30 vehicles parked near the gate of organisations offices.
SCIRI founder Ayatollah Muhammad Baqr al-Hakim was killed in a deadly car bombing in the Shiite holy city of Najaf last year. His brother now heads the outfit.
Al Hakim, head of the Supreme Council for the Islamic Revolution in Iraq was at his residence in Baghdad when the blast took place, said his spokesman, Haitham al-Husseini.
"It was a suicide attack near the gate leading to the office," al-Husseini
Al Hakim also heads the candidate list of the 228-member United Iraqi Alliance coalition, which is expected to dominate Iraq's new constitutional assembly following the country's January elections.
The United Iraqi Alliance coalition is backed by Iraq's top Shia cleric, Grand Ayatollah Ali al-Sistani.
Al Hakim's residence cum office was previously the house of Tariq Aziz, a senior aide to the former president Saddam Hussein, jailed since April 2003.
Shias, who comprise almost 60 per cent of the Iraqi population, were dominated by the Sunni minority during Saddam Hussein's regime. The attacks on the community's leaders have intensified since it expressed support for the elections.