Cairo (CNN) -- Bloodshed intermingled with blood oaths early Wednesday in Egypt, as time appeared to be running out for Egyptian President Mohamed Morsy.
Health officials have counted the bodies of 23 people who died when Morsy's angry opponents met head-on overnight with his supporters at Cairo University.
Hundreds more left the clashes with bruises and other injuries, state-funded Al-Ahram news reported, after rocks, Molotov cocktails and bullets flew.
Demonstrations elsewhere in Egypt were mostly peaceful, and protest leaders have called for nonviolence.
But as a deadline approached on an ultimatum the military gave Morsy this week, the leaders spoke of potential bloodshed.
"We swear by God that we are ready to sacrifice our blood for Egypt and its people against any terrorist, extremist or ignorant," they said in a statement, which was titled "The Final Hours."
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On Monday, the military gave Morsy 48 hours to accommodate his opponents with a power sharing agreement or be pushed aside. That leaves him until about 5 p.m. local time (11 a.m. ET) Wednesday.
He has already said that he will not comply.
The head of the Egyptian army, Gen. Abdul Fattah Al-Sisi, called an emergency meeting with commanding officers hours ahead of the deadline, state TV reported.
In the meantime, an opposition spokesman took potshots at Morsy at a news conference. He accused the United States of propping him up 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 Wednesday, saying Morsy should be charged with treason.
"Not America, not Morsy, not anyone can impose their will on the Egyptian people," Badr said.
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With the ultimatum, the armed forces appear to have thrown their weight behind those voicing their vehement opposition to Morsy's Islamist government, and they have joined them in the streets.
Early Wednesday, soldiers and police set up a perimeter around their central meeting point, Cairo's Tahrir Square, "to secure it from any possible attack," the state-run EgyNews agency reported.
In a twist of irony, it was the police force that, on the same spot in 2011, killed hundreds when they fired upon democratic, moderate and Islamic demonstrators, who stood side by side to overthrow former autocratic President Hosni Mubarak.
Mubarak had long repressed the Muslim Brotherhood, an Islamic political movement that it is now the nation's most powerful political force.
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The democratic reformists and moderates say Morsy's government has tightened its grip on power, moving in an authoritarian direction. Now they have joined forces with Mubarak's followers and citizens yearning for the restoration of order through the military's iron hand.
Together they are pushing hard to oust Morsy and his Islamist government, which was mainly formed from the ranks of the Muslim Brotherhood. They say they have collected 17 million signatures on a petition to remove him -- 4 million more than the number who voted Morsy into the presidency.
The most zealous anti-Morsy demonstrators have ransacked Muslim Brotherhood offices all over the country in recent days, torching at least one.
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.
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Coup or no?
Military leaders have told Arab media that they plan to suspend the constitution, dissolve the parliament and sideline Morsy, should he remain unbending.
In his place, they would install a mainly civilian interim council, until a new constitution can be drafted and a new president elected.
But military leaders have distanced themselves from the word "coup."
Their ultimatum was meant to push all factions toward a national consensus; the armed forces aren't looking to be part of the political or ruling circles, a spokesman, Col. Ahmed Ali, said Monday in a written statement.
The military appears to be pressuring Morsy to restructure his government to reduce the influence of the Muslim Brotherhood and include opposition members, a source close to highly placed members of Egypt's leadership told CNN.
Morsy's ministers would seem to have made a reshuffle easy on him, as five of them resigned this week, including Foreign Minister Mohamed Kamel Amr.
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Potential for violence
Like the opposition, Morsy's supporters are numerous and adamant.
They believe in the legitimacy of their government and feel their opponents are circumventing the democratic process by trying to depose their elected government.
In a televised speech late Tuesday in reaction to the military's ultimatum, Morsy reiterated that position.
"The people of Egypt gave me the mandate as president. They chose me in a free election. The people created a constitution," he said. "I have no choice but to bear responsibility for the Egyptian Constitution."
He demanded the military withdraw its ultimatum and return to its barracks.
The unrest prompted U.S. President Barack Obama to call Morsy on Monday and urge a less rigid stance. "He stressed that democracy is about more than elections," a White House statement said.
He pushed him to form a more inclusive government.
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On Tuesday, Obama called on Morsy to hold early elections, a senior administration official said. But he has not said he should step down immediately.
"We are saying to him, 'Figure out a way to go for new elections,' " the official said. "That may be the only way that this confrontation can be resolved."
A State Department spokeswoman, however, denied that Obama urged early elections.
Washington has also warned Egypt's military that it could lose millions in aid, if it carries out a coup.
Though Muslim Brotherhood leaders have called members to refrain from bloodshed, others have warned them to be prepared to die.
And one Islamist group said it would take up arms if Morsy is deposed.
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Morsy, a U.S.-educated religious conservative, was elected Egypt's president in June 2012. But his approval ratings have plummeted as Egyptian society and its economy have spiraled down.
His government has failed to keep order as crime has soared, including open sexual assaults on women in Egypt's streets. The chaos after the uprising has driven many tourists and investors away.
That has disaffected many of his supporters among Egypt's poor and middle classes, said Fawaz Gerges, director of the Middle East Center at the London School of Economics.
"The millions of Egyptians who cheered for Morsy are saying he must go," Gerges said.
He called Morsy "incompetent" but said he doubted the military would depose him.
That would drive Egypt into an even deeper crisis.
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