Despite the ceasefire, al-Sadr's followers have clashed with Iraqi government troops and US forces in the south of the country and Baghdad in recent weeks, leading to Iraq's worst violence since the first half of 2007.
Al-Sadr also indefinitely postponed an anti-US rally scheduled to mark the fifth anniversary of the US's capture of Baghdad.
The Shia leader said he feared his supporters would be attacked if the protest went ahead.
"I call those beloved Iraqi people who wish to demonstrate against the occupation to postpone their march, out of my fear for them and my concern to spare their blood," he said.
"I fear that Iraqi hands will be lifted against you, although I would be honoured if the Americans were to lift their hands against you."
Al-Sadr had called for a "million-strong" protest to mark the fifth anniversary on Wednesday of the fall of Baghdad.
Ahead of the anniversary, Iraqi authorities on Tuesday imposed travel restrictions into Baghdad to keep men aged between 12 to 35 from entering the city from 6am (0300 GMT) to 6am on April 10.
Show of force
Many analysts saw al-Sadr's planned protest as a show of force against the government, which has threatened to have al-Sadr's followers banned from participating in politics or the provincial elections.
If his movement does compete in elections later this year, al-Sadr stands to make significant gains.
The Shia leader's supporters have said that the Mahdi army will only be dismissed if recommended by Grand Ayatollah Ali al-Sistani and other Shia religious leaders, who have so far remained silent.
Meanwhile, at least four mortars hit the government and diplomatic Green Zone on Tuesday, while a roadside bomb targeting a police patrol near al-Shaab stadium in the east of the capital exploded, wounding two policemen, police sources said.
Later, another roadside bomb hit a US patrol near Sadr City, injuring a number of soldiers, police said.
Sporadic gunfire and explosions were heard across Baghdad's Sadr City district.
Iran condemns attacks
For the first time, Iran's foreign ministry condemned rocket and mortar attacks by the Mahdi army against the Green Zone in Baghdad.
But Mohammad Ali Hosseini, Iran's foreign ministry spokesman, also denounced raids by US forces in Sadr City, a Mahdi army stronghold.
"We are hopeful that restraint and prudence of various Iraqi groups will provide security and peace," Hosseini was quoted as saying on the state broadcasting company's website.
Iran has been accused of supplying weapons, money and training to most Iraqi Shia factions, including al-Sadr loyalists.
In other violence on Tuesday, at least six people are reported to have died in a roadside bomb attack on a bus in Bala Druz in Diyala province on Tuesday.
Also in Diyala, armed men killed Sheikh Sami al-Ubaidi, an awakening group leader, and his two sons.
In Washington, General David Petraeus, the highest US commander in Iraq, was due to report to congress on the progress of the US "surge" in Iraq later on Tuesday.
He is expected to call for patience and announce that he will stop plans for troop withdrawals in July.