Other revolutionaries rejected the return of military rule, even temporarily.
Morsi has to save himself and save the country and resign, said Maher of the 6th of April Movement.
The military statements marked another tumultuous turn of events in what has been more than two years of turmoil, as Egypt transitioned from six decades under civilian-military partnership to a contentious 18-month period under military rule to a year under a democratically elected civilian government to a period of uncertainty.
Perhaps the most succinct description of events over the past two years here came in the form of a tweet: Deja coup.
Egypts current constitution allows for a president to be removed only by a legislative referendum. But there is no legislature; after the countrys Supreme Court ordered the lower house of parliament dissolved over election irregularities, Morsi gave himself legislative powers until an election could be held in the fall. Under Egypts new constitution, passed in December, Egypt has no vice president.
Any power that refrains from the constitution is representing a call for vandalism and chaos, Yaser Hamza, a leader of the Muslim Brotherhood-affiliated Freedom and Justice Party, told the Al Ahram newspaper.
The militarys statement came after an unprecedented number of Egyptians turned out Sunday at protests timed to coincide with the first anniversary of Morsis inauguration. Some officials estimates of the crowds put their number as high as 14 million, or one of every six Egyptians.
While largely peaceful, violence erupted in the late hours of the protests at the Muslim Brotherhood headquarters in Cairo, where eight people died, according to government figures. Witnesses said protesters threw Molotov cocktails at the building and those inside responded with gunfire. Neither the police nor the military intervened.
Another six were killed around the country and 757 were injured, according to Health Ministry statements.
Morsi once enjoyed a 75 percent approval rating, but that fell precipitously to 23 percent over the last year, amid a collapsing economy, a polarized political situation and an Egypt increasingly defined and led by the Brotherhood.
The turning point came last fall, when he declared that his decisions were above judicial review and his backers hurriedly passed a constitution. There were deadly clashes in front of the presidential palace then, and Morsi appeared to have lost the majoritys support.
But for all the euphoria on the streets, there was no promise that the militarys ultimatum would deliver the peoples demand for stability or the hope of an improving economy. Many remembered that the economy and the political situation were not much better during the 18 months between Mubarak and Morsi, when the military ruled, and a prolonged period of uncertainty was unlikely to bring back investment or tourists.