The spokesman quoted al-Sadr as telling supporters at Imam Ali Mosque on Friday: "I advise the dictatorial, agent government to resign... the whole Iraqi people demands the resignation of the government... they replaced Saddam with a government worse than him."
Earlier, al-Sadr laid down a list of conditions for an end to clashes between his al-Mahdi Army and US-led occupation forces, according to a spokesman.
"The conditions set by al-Sadr are legitimate and reflect the popular will. We want to have a truce which is clear of deception and bluffing. Al-Sadr wants to protect the rights of the Iraqi people from the occupation and from the Iraqi government," Shaikh Ali Sumaisim told Aljazeera.
He spelled out the conditions - notably for the withdrawal of the US-led forces and the handover of Najaf to the Marjaayia, the Shia religious authority - during a news conference at a hotel in the besieged city in central Iraq.
If all multinational forces, Iraqi police and soldiers leave Najaf and the Marjaayia agrees to take responsibility for the city, "the Mahdi Army would pull out from Najaf", Sumaisim said, while stressing the militia would not disarm.
All basic services must be restored in Najaf, and al-Sadr's al-Mahdi Army should be recognised as an ideological movement with its members allowed to carry weapons for self-defence, Sumaisim continued.
Those jailed for supporting the resistance against the US-led occupation, all imprisoned clerics and women must also be released, the spokesman added.
Resistance fighters must no longer be persecuted and al-Sadr's organisation should be allowed to decide for itself whether it becomes a political movement, under the conditions.
"All followers of al-Sadr's movement should be under a legitimate constitution written by a free, elected government," Sumaisim said.
"Lastly, all efforts should be aimed at building a free, independent, unified Iraq," he added.
Meanwhile, an al-Mahdi Army leader described al-Sadr's health as satisfactory after he was lightly injured early on Friday after US forces bombarded the area.