Sadr said the proposed Status of Forces Agreement aimed at giving a legal basis to US troops after the December 31 expiry of a UN mandate defining their current status, was "against Iraqi national interests".
"After every Friday prayers, everyone must protest and demonstrate until the agreement is cancelled," he said in a statement sent to AFP on Wednesday.
He said his powerful movement planned to send delegations to neighbouring countries and to the West and to international bodies such as the United Nations, the Arab League and the European Ubnion to explain its position.
He said the Iraqi government should consult the people before entering into any deal with Washington.
US President George W. Bush and Iraqi Prime Minister Nuri al-Maliki signed a non-binding statement of principles in November for the negotiations, which began in March with the aim of concluding a pact by the end of July.
There are currently about 150,000 US troops deployed in Iraq, which was invaded by US-led forces in March 2003.
The proposed pact has come under fire from other religious and political leaders both in Iraq and in neighbouring Iran.
Fighters from Sadr's Mahdi army militia fought deadly street battles with US forces in the Shiite slum bastion of Sadr City in Baghdad for seven weeks until a May 10 truce deal took effect.
Iraq's national security council on Monday asked Maliki to ensure that the pact will not harm the national interest.
© 2008 Agence France Presse