Article published on the 2008-10-18 Latest update 2008-10-18 15:00 TU
In a written statement, Sadr, who is reputed to be living in Iran, told MPs that "the destiny of Iraq is in your hands".
"Do not vote for the agreement," the statement said. "If they tell you the agreement ends the occupation ... no, the occupier will still remain. If you are told that it would give sovereignty to Iraq, it is a lie."
The pact will be put to the Iraqi parliament after being signed by the two countries' presidents.
Details have not been made public but officials say that there is agreement that US combat troops will have left the country by 2011.
During months of negotiation, a principal point of contention has been whether American troops and contractors would be subject to US jurisdiction if accused of serious crimes in Iraq.
US Defence Secretary Robert gates said Friday that "our men and women in Iraq are well protected".
Iraqi Foreign Minister Hoshyar Zebari said on Saturday that it is time to decide on the deal.
"It is difficult to reopen the text," he said. "The parliament either ratifies or rejects it."
Sadr supporters claim that as many as a million people joined today's demonstration. It started in the movement's stronghold, Sadr City in east Baghdad, and ended at Mustansiriyah Square, where effigies of US President George Bush and Secretary of State Condeleezza Rice were burnt.
"It’s not clear how much strength the Sadrists have at the moment," says British-based journalist Patrick Cocburn, who has written a book on Sadr."Earlier this year they dominated about half Baghdad and about 80 per cent of Shia Baghdad."
Cockburn points out that Prime Minster Nouri al-Maliki has also blocked agreement on nationalist grounds.
"He refused to sign the original agreement, saying it gave the Americans too much leaway, gave immunity to their troops," he told RFI.
The protest was originally scheduled for 9 April, the anniversary of the fall of Saddam Hussein, but was postponed after clashes between Sadr's Mahdi Army militia and US and Iraqi troops.