CAIRO — Egypt was on edge today following a “last-chance” ultimatum the military issued to Mohammed Morsi, giving the president and the opposition 48 hours to resolve the crisis in the country or have the army step in with its own plan.

Protesters seeking the ouster of the Islamist president remained camped out at Cairo’s Tahrir Square, the birthplace of the 2011 uprising, gearing up for a third day of anti-Morsi rallies.

Across town, Morsi’s Islamist backers have hunkered down at their own rally site, vowing to resist what they depict as a threat of a coup against a legitimately elected president.

The military’s ultimatum, read Monday on state TV, put enormous pressure on Morsi to step down and sent giant crowds opposing the president in Cairo and other cities into delirious celebrations of singing, dancing and fireworks.

But it also raised worries on both sides that the army could take over outright as it did after the 2011 ouster of autocrat Hosni Mubarak and raised the risk of a backlash from Morsi’s Islamist backers, including his powerful Muslim Brotherhood and hard-liners, some of whom once belonged to armed militant groups.

Pro-Morsi marches numbering in the several thousands began after nightfall Monday in a string of cities around the country, sparking clashes in some places. An alliance of the Brotherhood and Islamists read a statement at a televised conference calling on people to rally to prevent “any attempt to overturn” Morsi’s election a year ago.

After midnight, Morsi’s office issued a statement saying a “modern democratic state” was one of the main achievements of the anti-Mubarak uprising, adding, “With all its force, Egypt will not allow itself to be taken backward.”

While not bluntly rejecting the ultimatum, it said Morsi was still reviewing the military statement.

President Barack Obama said the U.S. is committed to democracy in Egypt, not any particular leader. Traveling in Tanzania, Obama said that although Morsi was democratically elected, the government must respect its opposition and minority groups.

Egypt’s presidency said Morsi received a phone call from Obama, who said the U.S. administration “supports peaceful democratic transition in Egypt.”

The military’s statement came on the second straight day of anti-Morsi protests nationwide, and even though many of the opposition supporters welcomed it, it triggered echoes of a time when the generals were in power following Mubarak’s ouster.

Many of those now in the anti-Morsi campaign then led demonstrations against military rule, angered by its management of the transition and heavy hand in the killing of protesters.

Hours after its announcement, the military issued a second statement on its Facebook page denying it intended a coup.