Sheikh Salah Obaidi, spokesman for al-Sadr's bloc in parliament, said the call for protests is not a "threat" to the Iraqi government, but a "warning".
Al-Sadr on Tuesday warned the government against signing the agreement, saying "it is against the interests of the Iraqi people".
Abdul-Aziz al-Hakim, another key Shia leader, spoke out against the agreement, saying it would violate Iraq's sovereignty.
Last week Grand Ayatollah Ali al-Sistani, Iraq's most revered Shia cleric, also reportedly expressed his anger, saying he would not permit the Iraqi government to sign a deal with "US occupiers" as long as he lived.
'No permanent bases'
The US government has said it will not seek permanent bases in Iraq.
David Satterfield, a senior advisor on Iraq at the US state department, told Al Jazeera that the so-called Status of Forces agreement (Sofa) with Baghdad would address the issue.
"The Sofa agreement and the strategic frameworks agreement will make explicit that there is no desire for, indeed there is a rejection of permanent bases. We could not be clearer on this point," he said.
"We do not believe that there is a need for such bases."
Satterfield said that the US was conducting the negotiations with representatives of all the main leaders in Iraq.
Samir al-Sumaida'ie, the Iraqi ambassador to the US, said the agreement was not intended to "tie the hands of any future government in Iraq or any future administration in the US".
"It's a matter of the current need of Iraq," he said.
"Our obligation to our people is to protect their interests, the obligation of the US government is to protect the Americans.
"Where these interests coincide, then we can reach agreement."
Any prospective agreement can also be terminated two years after either party decides to do so, al-Sumaida'ie said.
However, in the US, the House of Representatives has adopted a bipartisan amendment requiring congressional approval for any proposed military accord with Iraq.
The move could prevent George Bush from approving the deal only months before leaving office.
Source: Al Jazeera