(CBS News) CAIRO, Egypt -- Egypt's army gave President Mohammed Morsi 48 hours on Monday to resolve a growing crisis, sahing it will step in if he doesn't resolve it.
The situation is so serious that Gen. Martin Dempsey, the top U.S. military officer, called his Egyptian counterpart to discuss it on Monday.
Protests have been building for weeks against Morsi, who became Egypt's first freely-elected leader just a year ago. Hundreds of thousands have taken to the streets. So have Morsi's supporters.
The ultimatum came as army helicopters flying the Egyptian flag circled overhead, prompting roars of approval from the hundreds of thousands of protesters in Tahrir square.
The protesters blame President Morsi and his main backers, the Muslim Brotherhood, for mismanaging the economy, and for the breakdown of security.
"Step down, step down," they chanted.
Muslim Brotherhood spokesman Gihad al Heddad told us that Morsi inherited Egypt's troubles from former president Hosni Mubarak.
"If you get a Formula 1 racer into a car with four flat tires, no steering wheel and an empty tank, it's not going to drive. It needs to be fixed first," said Gihad al Heddad.
It was Morsi's one year anniversary that prompted the mass protests.
Demonstrators claim Morsi has rewarded his Muslim Brotherhood supporters with key positions of power.
Sixteen people were killed in clashes in the last 24 hours, the majority in an attack on the Muslim Brotherhood headquarters.
An American student was killed in earlier violence on Friday.
Protesters were pelting the building with Molotov cocktails all night.
There are still some small fires burning. They covered the walls with graffiti that has become the rallying cry of this anti-Morsi movement - 'irhaal,' meaning 'leave.'
With the army's ultimatum, Morsi supporters are bracing themselves for a battle to defend their president, who they say has a right to finish his term.
"He has four years he can prove he can do it or he can't. If he can't, the people are going to choose someone else," said al Heddad.