Pleading for calm, U.N. High Commissioner for Human Rights Navi Pillay asked for an investigation into the violence.
"The number of people killed or injured, even according to the government's figures, point to an excessive, even extreme, use of force against demonstrators," she said. "There must be an independent, impartial, effective and credible investigation of the conduct of the security forces. Anyone found guilty of wrongdoing should be held to account."
The U.N. Security Council convened an urgent meeting Thursday evening on the crisis.
"It is important ... that the parties exercise maximum restraint, and there was a common desire to stop the violence and advance national reconciliation," Argentine U.N. Ambassador Maria Cristina Perceval said on behalf of the council members.
Germany, France and other nations summoned Egypt's ambassadors to their nations to express dismay over the violence, with Italy typical among them in criticizing the "force used by police (as) brutal, disproportionate and ... not justifiable."
Denmark suspended economic aid to the country. As to the United States -- which provides about $1.6 billion in annual aid to Egypt -- it is continuing to review such programs "in all forms," State Department spokeswoman Jen Psaki said Thursday.
Even predominantly Muslim nations voiced displeasure, with Turkey recalling its ambassador in Egypt to return to Ankara in light of the crisis, a Turkish foreign ministry spokesperson said.
Raids followed by chaos, attacks on churches
The violence erupted as security forces raided pro-Morsy camps Wednesday after weeks of simmering tension. Clashes and gunfire broke out, leaving pools of blood and bodies strewn in the streets.
Authorities bulldozed tents and escorted hundreds of people away. Some mothers and fathers managed to whisk away their children, gas masks on their faces.
Morsy supporters also reportedly attacked a number of Christian churches. It's not clear how many were targeted, but Dalia Ziada, of the Ibn Khadun Center for Development Studies, said Thursday that her group had documented the burning of 29 churches and Coptic facilities across the country.
"This is horrible to happen in only one day," she said.
The Bible Society of Egypt said 15 churches and three Christian schools had been attacked, some set on fire.
At least 84 people, including Muslim Brotherhood members, have been referred to military prosecutors for charges including murder and the burning of churches, the state-run EGYNews site reported.
Nile TV said Morsy supporters attacked another church Thursday in Fayoum, southwest of Cairo.
Muslim Brotherhood leader defiant
Rather than uniting Egypt after Mubarak's fall, divisions intensified during Morsy's time as president.
Critics accused him of being authoritarian, trying to force the Muslim Brotherhood's Islamic agenda on the country and failing to deliver freedom and justice.
Morsy's supporters say the deposed president wasn't given a fair chance and say his backers have been unfairly targeted for expressing their opinion.
Though Morsy has not appeared in public since he was taken into custody, his supporters have amassed on the streets nationwide to slam military leaders and demand his reinstatement.