Clashes have been reported between supporters of Sadr and rival factions in Al-Najaf, Baghdad, Al-Basrah, Nasiriyah, and other locations.
The unrest came as parliament was expected to vote later today on a draft constitution. The draft is opposed by Sadr and Sunni Muslim groups. Talks have been taking place aimed at winning Sunni backing for the document.
U.S. officials have meanwhile announced that an extra 1,500 American troops will be sent to Iraq to join the current U.S. force of around 138,000 soldiers in a bid to boost security for October's planned constitutional referendum.
For RFE/RL's full coverage of developments in Iraq, see "The New Iraq"