At least 100 people were killed early Saturday when riot police fired on protesters supporting deposed Egyptian President Mohammed Morsi in Cairo, activists said.
The Anti-Coup Alliance, an umbrella coalition of Morsi supporters, also said at least 5,000 people were injured near the clash in the Nasr City neighborhood of the capital.
Many of the bodies of those killed were still at a field hospital near the pro-Morsi protest site and were not officially registered with health officials. The official death toll was based only on bodies tallied at government morgues, so that figure was only at 21.
Hundreds of thousands of people took part in rival political rallies in various Egyptian cities on Friday and the violence continued into the night.
Both supporters of the army and Morsi backers had announced they would be protesting Friday and warned there could be bloodshed. Army chief Gen. Abdel Fattah al-Sisi urged Egyptians earlier in the week to hold rallies to give the military a “mandate” to confront weeks of violence caused by his July 3 overthrow of Morsi, Reuters reported.
"Sisi out! Morsi is president! Down with the army!" a gigantic crowd of Morsi protesters, who were holding a sit-in protests around Cairo's Rabaa al-Adawiya mosque, chanted, reported the BBC.
Morsi, who was elected by a narrow margin in June 2012 in the country's first democratic vote, has been hidden from the public since he was ejected from power. On Friday, Mena news agency reported he would be detained for 15 days while a judge investigated allegations against him. This is his first formal detention since he was ousted, Al Jazeera reported.
According to Reuters, the probe stems from charges that he conspired with Palestinian Islamist group Hamas to escape jail during the 2011 uprising against former President Hosni Mubarak, killing prisoners and officers, kidnapping soldiers, and lighting buildings on fire.
"At the end of the day, we know all of these charges are nothing more than the fantasy of a few army generals and a military dictatorship," Brotherhood spokesman Gehad El-Haddad said. "We are continuing our protests on the streets."
State Department spokeswoman Jen Psaki said the U.S. was "deeply concerned" by reports that Morsi had been ordered to be detained for more than two weeks.
"We of course are deeply concerned by reports that an Egyptian court has ordered the detention of Mr. Morsi," she said. "I can't speak to the specific charges, but we do believe that it is important that there be a process to work towards his release. Clearly this process should respect the personal security of him and take into account the volatile political situation in Egypt, and that's where our focus is."
A Reuters witness said thousands of pro-Morsi activists clashed with pro-army protesters in the Egyptian city of Alexandria, with some demonstrators hurling stones down on the crowds from nearby rooftops.
The army has warned it will “turn its guns” on those who use violence in the protests, Reuters reported, while the Brotherhood has said civil war is a possibility.
On Friday morning, several hundred pro-army supporters gathered in Tahrir Square, the central rallying place two years ago for Egyptians. By night, after the evening prayer marking the end of the day’s Ramadan fast, the crowd had grown to tens of thousands, if not hundreds of thousands, in the square, according to the BBC.
A banner across one entrance to Tahrir Square read, "The people, the source of all power, mandate the army and police to purge terrorism," the BBC said.
Pro-army demonstrations were planned throughout Egypt. Morsi backers had also announced a large amount of rallies, with 34 in the Cairo area alone, Reuters said.
UN leader Ban Ki-moon called on Egyptians to protest peacefully and asked the interim leadership to ensure security, reported Al Jazeera.
But casualties appeared likely after a month in which nearly 200 people, mostly supporters of Morsi, have died, according to Al Jazeera.
The West has become increasingly alarmed by the course the country of 84 million people has taken. Washington this week said it delayed delivery of four F-16 fighter jets to Cairo and called on the Egyptian army to exercise "maximum restraint and caution" during Friday's rallies," reported Reuters.
The U.S. has not made a determination on whether a military coup took place in Egypt, which avoids a decision on whether U.S. aid should be cut off to Egypt.
"It is not in our national interest to make such a determination," State Department spokeswoman Psaki said on Friday, adding the law didn't require that a formal determination be made.
If the administration were to determine a coup had occurred when Morsi was ousted, the $1.5 billion of military and economic assistance the U.S. provides Egypt would be required by law to stop.
NBC's Catherine Chomiak and Reuters contributed to this report.