U.S. forces detained Amar al-Hakim at a border checkpoint as his convoy returned from Iran on February 23.
Amar's father, Abd al-Aziz al-Hakim, heads the Supreme Council for the Islamic Revolution in Iraq (SCIRI), the biggest party in Iraq's government.
The U.S. military said al-Hakim and some of his bodyguards were initially detained because some of them behaved suspiciously at a border crossing used by weapons smugglers. The military described the incident as "unfortunate".
U.S. Ambassador Zalmay Khalilzad apologized, saying he did not know the reason for the detention.
Amar al-Hakim said he was treated rudely by U.S. forces.
An estimated 8,000 Shi'a protested in Al-Najaf and several other southern cities today.
(compiled from agency reports)