In the wake of violent clashes that killed dozens, Secretary of State John Kerry said Egypt is at "a pivotal moment" more than two years since the uprising ousted the longtime President Hosni Mubarak.
In a statement released Saturday, Kerry said Egyptian officials "have a moral and legal obligation" to respect the right of peaceful assembly and freedom of expression. He added that the continued violence sets back efforts of "reconciliation and democratization," and affects regional stability.
Also on Saturday, Secretary of Defense Chuck Hagel spoke by phone with Army chief Gen. Abdel Fattah al-Sisi to express deep concern about the security situation and to encourage that restraint be exercised during this "difficult period."
"The United States believes that the current transition needs to be marked by inclusivity, that Egyptian authorities should avoid politicized arrests and detentions, and take steps to prevent further bloodshed and loss of life," Pentagon press secretary George Little said.
Dozens of people were killed early Saturday when riot police fired on protesters supporting deposed Egyptian President Mohammed Morsi in Cairo, activists said.
It was not clear exactly how many people were dead Saturday evening, but the Anti-Coup Alliance, an umbrella coalition of Morsi supporters, said that at least 100 people were killed and 5,000 others were injured in the chaotic clash near 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 of 21 was based only on bodies tallied at backlogged government morgues.
If the death toll of 100 or more is confirmed, it would make the clash the most lethal incident since Morsi was ousted by the military July 3. It comes nearly three weeks after over 50 people, primarily Muslim Brotherhood supporters, died in a similar outburst of bloodshed.
The violence reportedly started after hundreds of Morsi supporters moved out of their encampment outside of the Rabaah al-Adawiyah Mosque late Friday. One group began to pitch tents on an adjoining boulevard, where they planned to camp out for at least three days, said Mahmoud Zaqzouq, a Muslim Brotherhood spokesman, according to The Associated Press.
Meanwhile, Zaqzouq said, another group of protesters moved toward a nearby overpass, where they were met with a barrage of tear gas from police forces, followed by hails of bullets. Protesters counterattacked by throwing rocks and stones at the security forces, according to the AP.
"There were snipers on the rooftops, I could hear the bullets whizzing past me," Ahmed el Nashar, 34, told Reuters, fighting back tears.
"Man, people were just dropping."
Doctors at a nearby makeshift hospital were flooded by a deluge of casualties as activists rushed bodies away from the carnage, according to The Associated Press. The majority of the dead were struck in the head, some between the eyes, according to Reuters.
Hundreds of people participated in rival rallies in various Egyptian cities on Friday, just days after al-Sisi urged Egyptians to hold rallies to give the military a “mandate” to confront weeks of violence triggered by his overthrow of Morsi, according to Reuters.
But Brotherhood activists told Reuters they would not back down and warned of even more violence if security forces did not relent.
"We will stay here until we die, one by one," Ahmed Ali, 24, told Reuters, as he helped tend to casualties at the field hospital.
"We have the examples of Algeria and Syria in our minds. We don't want it to become a civil war. If we take up arms it might become one. This is a religious belief," he said.
Throngs of Brotherhood-aligned activists were massed in downtown Cairo on Friday evening, where they chanted pro-Morsi slogans, some waving the Egyptian flag.
Morsi under investigation
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. Mena news agency reported Friday that he would be detained for 15 days while a judge reviewed allegations against him. It marks the first formal detention since he was deposed, according to Al Jazeera.
According to Reuters, the probe stems from charges that he conspired with Palestinian Islamist group Hamas to escape jail during the 2011 uprising against 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,” El-Haddad said, according to Reuters. “We are continuing our protests on the streets.”
The U.S. was “deeply concerned” by reports that Morsi had been ordered to be detained for more than two weeks, said State Department spokeswoman Jen Psaki.
"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."
Nearly 200 people, mostly supporters of Morsi, have died in the last month, 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.
Catherine Chomiak and Elizabeth Chuck of NBC News and Reuters contributed to this report.