CAIRO (CNN) -- Bloodshed intermingled with blood oaths early Wednesday in Egypt.
Clashes killed at least 16 people, when angry opponents and supporters of President Mohamed Morsy met head on at Cairo University, according to the state-run EgyNews agency. Another 200 suffered injuries.
Leaders of Egypt's army vowed to "sacrifice our blood," to defend the country, just hours after Morsy said he would not bow to their ultimatum to come up with a power sharing agreement.
On Monday, the military gave the Egyptian president 48 hours to respond to the demands of the opposition or be pushed aside.
With the ultimatum, the armed forces appear to have thrown their weight behind those voicing their vehement opposition to Morsy's government.
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," EgyNews reported.
In a twist of irony, it was the same police force that, on the same spot in 2011, fired upon democratic, moderate and Islamist demonstrators, killing hundreds, as they fought to overthrow former autocratic President Hosni Mubarak, himself once a military commander.
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.
The opposition is pushing hard to ouster Morsy and his Islamist government, which was mainly formed from the ranks of the Muslim Brotherhood. Zealous anti-government demonstrators have ransacked and sometimes torched MB offices all over the country in recent days.
Some of ithe group's leaders have complained that the 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.
Time may be running out for Morsy, as military leaders have said they plan to suspend the constitution, dissolve the parliament and sideline Morsy, military sources told Arab media.
In his place, the military has said it would install an interim council, made up mainly of civilians, until a new constitution can be drafted and a new president elected, the sources said.
They have backed up their plans with blood curdling rhetoric.
"We swear by God that we are ready to sacrifice our blood for Egypt and its people against any terrorist, extremist or fool," they said in a statement, which was titled "The Final Hours."
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.
The steps could include reducing the influence of the Muslim Brotherhood in his Cabinet and calling early presidential and parliamentary elections, a source close to highly placed members of Egypt's leadership told CNN.
Morsy's ministers would seem to have made a restructuring easy on him, as many of them have resigned in recent days, including Foreign Minister Mohamed Kamel Amr.
Like the opposition, Morsy's supporters are numerous and adamant. Some of their leaders have warned them to be prepared to die. Others are admonishing members to refrain from violence.
"We know that the voice of peace is louder than that of the gun," MB leader Abdel-Rahman El-Bar told thousands of Morsy supporters days ago. "We will not shed blood."
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 the stance.
"The people of Egypt gave me the mandate for president. They chose me in a free election. The people created a constitution which requires me to stay with the constitution," he said. "I have no choice but to bear responsibility for the Egyptian constitution."
Morsy gave no indication of stepping down, sharing power or calling early elections. He demanded the military withdraw its ultimatum and return to its rightful work.
The unrest prompted U.S. President Barack Obama to call Morsy Monday. He confronted the leader with the rigid stance. "He stressed that democracy is about more than elections," a White Hous statement said.
On Tuesday Obama called on Morsy to hold early elections but has not said he should step down immediately.
"We are saying to him, 'Figure out a way to go for new elections,'" a senior Obama administration official said. "That may be the only way that this confrontation can be resolved."
Washington has also warned Egypt's military that it could lose millions in aid, if it carries out a coup.
Morsy, a U.S.-educated Islamist, was elected Egypt's president in June 2012, but critics say he has become increasingly authoritarian during his year in power.
His government has failed to keep order, as crime has soared, including open sexual assaults on women in Egypt's streets. And he has failed to revive Egypt's economy, which crashed when the 2011 uprising that toppled Mubarak drove 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.
"That some of the revolutionaries are calling on the army to return to politics is a testament to how polarized Egypt is a year after the election of Morsy," Gerges said. "Think of the millions of people who cheered Morsy after his election. Think of the millions of Egyptians who pinned their hopes on Morsy.
"A year later, now, the millions of Egyptians who cheered for Morsy are saying he must go."
Gerges questioned Morsy's ability to continue to lead but said he doubted the military would depose him. Such a move "would plunge Egypt into a greater legal, political and institutional crisis," he said.
Mubarak had long repressed the Islamic political movement, but it is now the nation's most powerful political force.
Anti-government demonstrators say they have collected 17 million signatures -- 4 million more than the number who voted Morsy into the presidency -- calling for him to go.
CNN's Tom Watkins, Chelsea Carter, Jamie Crawford, Schams Elwazer, Karen Smith, Elise Labott, Ben Wedeman, Ian Lee, Housam Ahmed and Salma Abdelaziz contributed to this report.