Egypt's military's intelligence service placed a travel ban on President Mohamed Morsy and senior Islamist aides including the Muslim Brotherhood's Supreme Guide Mohamed Badie, and his influential deputy, Khairat el-Shater, security officials said.
An adviser to President Mohamed Morsy suggested that a military coup was under way in Cairo on Wednesday. Essam al Haddad, Egypt's presidential senior adviser on foreign affairs said on a government Facebook page: "As I write these lines I am fully aware that these may be the last lines I get to post on this page. For the sake of Egypt and for historical accuracy, let's call what is happening by its real name: Military coup."
Morsy offered to form an interim coalition government until parliamentary elections can be held and the constitution can be amended.
"The presidency's vision includes the formation of a coalition government that would manage the upcoming parliamentary electoral process, and the formation of an independent committee for constitutional amendments to submit to the upcoming parliament," he said in a posting on his Facebook page.
"The presidency blames for the most part a number of political parties that boycotted all calls for dialogue."
It added, "To protect the blood of Egyptians, the presidency calls on all political and national forces to prioritize national interest above all other interests."
Morsy noted that hundreds of thousands of supporters and protesters had packed plazas around the country.
"One of the mistakes I cannot accept -- as the president of all Egyptians -- is to side with one party over another, or to present the scene from one side only. To be fair, we need to listen to the voice of people in all squares."
He urged that his countrymen be allowed to express their opinions through the ballot box.
The posting came as the nation's first democratically elected president and his supporters were facing off against demonstrators who had packed public spaces around the country to demonstrate their opposition to his government.
Whether his statement would suffice to stave off military intervention was not immediately clear.
But an aide to Morsy, Essam al Haddad, said in a Facebook posting that a coup was under way.
"For the sake of Egypt and for historical accuracy, let's call what is happening by its real name: military coup," said al Haddad, who works in the office of the assistant to the president on foreign relations.
"Today, only one thing matters. In this day and age, no military coup can succeed in the face of sizable popular force without considerable bloodshed. Who among you is ready to shoulder that blame?"
He added, "In a democracy, there are simple consequences for the situation we see in Egypt: The president loses the next election or his party gets penalized in the upcoming parliamentary elections. Anything else is mob rule."
On Tuesday night, Morsy had vowed that he would not comply with an ultimatum delivered Monday by the military demanding that he enter into a power-sharing agreement.
"There is no alternative to the constitutional legitimacy and to the constitution," he said in a televised address. "I will not allow for anyone to say things that will violate this legitimacy or to take steps that will shake up it up."
He added: "If the price of upholding this legitimacy is my own blood, I am, therefore, ready to sacrifice my blood for this country and its stability."
He demanded the military withdraw its ultimatum and return to its barracks.
On Wednesday afternoon, the Interior Ministry said in a statement that it was determined to maintain order. "The police force stands side by side with the armed forces in protecting the country and the citizens," it said.
Reports of a TV studio takeover
Reuters and several other news organizations reported that Egyptian troops had "secured the central Cairo studios of state television" as the deadline approached and that staff not working on live shows had departed.
CNN has not confirmed the reports; state television denied in an on-air banner that there was any additional military presence at its studios.
Massive demonstrations for and against the former Muslim Brotherhood leader who was elected to office a year ago have been largely peaceful.
But 23 people died, health officials said, and hundreds more were injured in clashes overnight at Cairo University, the state-funded Al-Ahram news agency reported.
Protest leaders have called for nonviolence.
Egypt's military met Wednesday with religious, national, political and youth leaders to address the crisis, Egyptian military spokesman Ahmed Ali said through his Facebook page.
Hours earlier, an opposition spokesman accused the United States of propping up Morsy out of concern for neighboring Israel.
"The hour of victory is coming," said Mahmoud Badr of the Tamarod opposition group. He predicted that the "illegitimate president" would be gone by the end of the day.
"Not America, not Morsy, not anyone can impose their will on the Egyptian people," Badr said.
With the ultimatum, the armed forces appeared to have thrown their weight behind those opposed to Morsy's Islamic government.
Early Wednesday, soldiers and police set up a perimeter around the opposition's central meeting point, Cairo's Tahrir Square, "to secure it from any possible attack," the state-run EgyNews agency reported.
It was the police who, on the same spot in 2011, killed hundreds when they fired upon democratic, moderate and Islamic demonstrators seeking to overthrow Hosni Mubarak, the country's longtime autocratic leader and U.S. ally.
Mubarak had long repressed the Muslim Brotherhood, an Islamic political movement that emerged as the nation's most powerful political force once Mubarak was ousted.
Many of the democratic reformers and moderates who accused Morsy's government of moving in an authoritarian direction now support former Mubarak allies and others fed up with the nation's direction in calling for the restoration of order through the military.
They are pushing to oust Morsy and his Muslim conservative government, whose leaders were drawn primarily from the ranks of the Muslim Brotherhood. They say they have collected more than 20 million signatures on a petition to remove him -- millions more than the number who voted Morsy into the presidency.
In recent days, anti-Morsy demonstrators have ransacked Muslim Brotherhood offices all over the country.
Brotherhood members have complained that police did not protect them. Some have taken matters into their own hands, in one instance firing upon vandals with shotguns, an international journalism association reported.