CAIRO — Egypt entered a perilous 48 hours on Monday when the military delivered an ultimatum to the country’s first democratically elected president, hundreds of thousands of protesters renewed calls to oust him from office and the president’s Islamists allies vowed to take to the streets to stop what they called “a military coup.”

In a military communiqué read over state television that echoed the announcement toppling former President Hosni Mubarak two chaotic years ago, the Supreme Council of the Armed Forces demanded Monday that President Mohamed Morsi satisfy the public’s demands within two days, or else the generals would impose their own “road map” out of the crisis.

But instead of soothing the volatile standoff between Mr. Morsi’s opponents and his supporters, the generals seemed to add to the uncertainty that has paralyzed the state, decimated the economy and brought millions into the streets Sunday demanding the president step down. It was not clear what the military meant when it said Mr. Morsi must satisfy the public’s demands, what it might do if that vague standard was not met and who would be able to unite this badly fractured nation.