Egyptian medical workers at a field hospital in the capital, Cairo, said Saturday that at least 38 people have died in clashes between security forces and supporters of ousted President Mohamed Morsi.
Other reports say the death toll may be nearly twice that number.
Doctors said they are being overwhelmed at the hospital, which is operating from the sit-in in Cairo's Nasr City, a Muslim Brotherhood stronghold, where protesters have been camped for over three weeks.
On Nasr street, a main boulevard leading toward Rabaa and scene of Saturday's clashes, several make-shift brick walls have been built between, where security forces and emergency personnel are massed, and the edge of the protesters' encampment. It is unclear who built them. Streets are strewn with rubble and tear gas is still thick in the area.
Morsi supporters had massed Friday outside the Rabia el-Adawiya Mosque, while the Brotherhood tweeted photos of other pro-Morsi rallies around the country.
There have been reports of clashes between Morsi supporters and some of the thousands of anti-Morsi protesters gathered in Cairo's Tahrir Square.
Army chief Abdel Fattah el-Sissi called Friday for "all honorable Egyptians" to take to the streets to give him a mandate to fight what he called "violence and terrorism."
The demonstrations got under way as an Egyptian judge ordered the continued detention of ousted President Mohamed Morsi on conspiracy charges.
The official MENA news agency said Morsi is being detained for 15 days, while officials investigate charges he conspired with the Palestinian militant group Hamas. The Islamist leader has been held without charge in secret military detention since July 3, when he was removed by Egypt's army.
Morsi's presidency and his subsequent ouster has bitterly divided Egyptians.
World leaders are expressing increasing concern about the rising violence, the polarization of Egyptian society, and the army's crackdown on Brotherhood leaders.
On Thursday, United Nations Secretary-General Ban Ki-moon called for restraint on all sides, saying he supports the rights of all Egyptians to hold peaceful protests.
He also said the Egyptian military should "end arbitrary arrests and other reported forms of harassment." He said Morsi and other detained members of the Muslim Brotherhood should either be freed or have their cases reviewed.
The Egyptian military dominated Egyptian politics for decades, until the overthrow of president Hosni Mubarak, himself an ex-military commander. It has a long history of animosity toward the Muslim Brotherhood, which is Egypt's most organized political party.