The motorist was stopped late yesterday by US troops conducting search operations on a street in the centre of the central city of Kut, Lieutenant Mohamad Abdel Abbas said.
After the man refused to remove Sadr's picture from his car, the soldiers forced him out of the vehicle and started beating him with truncheons, he said.
US troops also detained from the same area five men wearing black pants and shirts, the usual attire of Sadr's Mehdi Army militiamen and followers.
Qassem Hassan, the director of Kut general hospital, identified the man as Salem Hassan, a resident of a Kut suburb.
He said the man had died of wounds sustained in the beating.
A spokesman for the US-led coalition could not confirm the incident.
Source: Agence France-Presse