The mass public show of opposition came as U.S. and Iraqi leaders face a Dec. 31 deadline to reach agreement on the deal, which would replace an expiring U.N. mandate authorizing the U.S.-led forces in Iraq.
Al-Sadr's message was addressed to Iraqi lawmakers and read by his aide Sheik Abdul-Hadi al-Mohammadawi before a huge crowd of mostly young men waving Iraqi and green Shiite flags and chanting slogans including "no, no to the agreement" and "yes to Iraq."
"The Iraqi government has abandoned its duty before God and its people and referred the agreement to you knowing that ratifying it will stigmatize Iraq and its government for years to come," he said.
Al-Sadr, who is living in Iran, also cast doubt on the Iraqi government's argument that the security pact is a step toward ending the U.S. presence in Iraq. The deal would require U.S. forces to leave by Dec. 31, 2011 unless Iraq asked some of them to stay.
"If they tell you that the agreement ends the presence of the occupation, let me tell you that the occupier will retain its bases. And whoever tells you that it gives us sovereignty is a liar," al-Sadr said. "I am confident that you brothers in parliament will champion the will of the people over that of the occupier ... Do not betray the people."
The demonstrators marched from the main Shiite district of Sadr City to the more central Mustansiriyah Square in eastern Baghdad.
"No, No to America," shouted one man, wearing a white Islamic robe as he sat in a wheelchair and clutched a poster of the Iraqi flag. "We prefer death to giving concessions."
Security was tight, with Iraqi security forces manning checkpoints on sidestreets and snipers on rooftops. Iraqi Humvees controlled all the roads leading to the square. Giant Iraqi flags covered nearby buildings.
One banner in English said "We refuse the existence of the U.S. in Iraq."
Prime Minister Nouri al-Maliki's government and the Bush administration have hammered out a draft agreement after months of bitter negotiations.
But the Iraqi parliament must ratify the deal and Iraq's pre-eminent cleric Grand Ayatollah Ali al-Sistani has said any accord must have national consensus.
Al-Maliki, a Shiite, could be politically isolated if he tries to win parliament's backing in the face of widespread opposition.
Several Sunni and Shiite clerics, who wield considerable influence in shaping public opinion, also spoke out during Friday prayer services against the draft, complaining that the Iraqi public knows little about the terms.
A copy of the draft accord obtained by The Associated Press specifies that U.S. troops must leave Iraqi cities by the end of June and be gone by 2012. It gives Iraq limited authority over off-duty, off-base U.S. soldiers who commit crimes.
U.S. Congressional approval is not required for the pact to take effect, but the administration is trying to build maximum political support anyway.
The march was called by al-Sadr after he had to postpone a mass demonstration on April 9 to mark the fifth anniversary of the U.S. capture of Baghdad. That march had been postponed after many of his followers complained they were not allowed to enter the capital amid fears of violence.
