Speaking to tens of thousands people at a rally at the Rabaa al-Adawiya mosque in Cairo, Safwat Hegazy said the army would return Morsi to power "whether they want to or not," Ahram Online reported.
Morsi supporters were willing to "die as martyrs," Hegazy said.
Morsi backers from Cairo and cities around the country began gathering at the mosque Thursday, concerned the army might block routes to the mosque in advance of the rally.
Egyptians who support the military coup that removed the president also called for mass demonstrations.
The National Salvation Front and other groups have called for Egyptians to gather at Tahrir Square and the presidential palace where they will celebrate the iftar meal, which is eaten after sunset during Ramadan.
The military said it opened fire during last week's deadly clashes only after being attacked with stones, gunfire, Molotov cocktails and even toilets hurled from rooftops.
At least 51 protesters were killed in that violence.
Army spokesman Ahmed Ali told reporters security forces were targeted with live ammunition.
Ali said the army first responded with tear gas, blank cartridges and rubber bullets, and only later "used live ammunition in non-deadly parts of the body."
But doctors who treated the wounded at a makeshift field hospital said many victims had neck, chest and head wounds, the Los Angeles Times reported.
Ali said the army's investigation concluded the assailants were "outlaws -- they are criminals ... this was not peaceful protest ... it was premeditated violence."
The Muslim Brotherhood, which has had many of its leaders arrested since Morsi's ouster, expressed outrage the army was trying to blame its supporters for the violence.
Supporters were called on to "continue our peaceful resistance to the bloody military coup against constitutional legitimacy," a statement from the organization said.
"We trust that the peaceful and popular will of the people shall triumph over force and oppression," the statement said.
The military-led interim government is expected to finish forming a Cabinet to replace Morsi's as early as Sunday, the Times said.
Interim Prime Minister Hazem el-Beblawi and other officials invited the Muslim Brotherhood's Freedom and Justice Party to join the coalition.
But party leaders rejected the offer, calling the interim government illegitimate.