Updated 12:05 p.m. ET
CAIRO With a for intervention ticking down, hundreds of thousands of protesters seeking the ouster of Egypt's Islamist president sought Tuesday to push the embattled leader further toward the edge with another massive show of resolve and unity.
In a significant move, opposition parties and the youth movement behind the demonstrations agreed that reform leader and Nobel Peace laureate Mohamed ElBaradei would represent them in any negotiations on the country's political future.
The pact, at least in theory, should end the bickering and rivalries that have plagued the opposition.
At the same time, President Mohammed Morsi faced fissures from within after a stunning surge of street rage reminiscent of Egypt's Arab Spring revolution in 2011 that cleared the way for Morsi's long-suppressed Muslim Brotherhood to win the first open elections in decades.
Three government spokesmen were the latest to quit as part of high-level defections that underscored his increasing isolation and fallout from the ultimatum from Egypt's powerful armed forces to either find a political solution by Wednesday or the generals would seek their own way to end the political chaos.
Gehad al-Haddad, a senior Muslim Brotherhood spokesman, told CBS News' Alex Ortiz the military's threat has changed the game.
"I think it's a coup," al-Haddad said.
Al-Haddad called the threat "completely unacceptable," but added that his organization will not be caught off-guard if the military makes good on its threat.
"We are shifting our tactics," al-Haddad said. "We have had a scenario for this for some time. If military moves on the ground we have a plan for that."
CBS News correspondent Clarissa Ward spoke with al-Haddad on Monday, and he told her 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-Haddad.
The police, which are under control of the Interior Ministry, have stood on the sidelines of the protests, refusing even to protect the offices of the Muslim Brotherhood that have been attacked and ransacked. Their ministry has thrown its considerable weight behind the military.
In response to the growing pressures on Morsi, his Islamist backers have stepped up their own warnings that it may take bloodshed to dislodge him.
Some supporters say they would rather die fighting a military takeover than accept Morsi's ouster just a year after the country's first free election.
"Seeking martyrdom to prevent the ongoing coup is what we can offer as a sign of gratitude to previous martyrs who died in the revolution," Brotherhood stalwart Mohammed el-Beltagi wrote Tuesday in his official Facebook page.
Morsi, meanwhile, met with Defense Minister and army chief Abdel-Fattah el-Sissi and Qandil in the second such meeting in as many days. No details were given about the meeting, reported by an official at the president's office, Ayman Ali.
The meeting, however, suggests that efforts were being made to resolve the crisis, although there is little time and almost no political will from the opposition to accept anything less than Morsi's departure.
At least 16 have been killed in clashes since Sunday between Morsi's opponents and his many backers, who have equated the demonstrations and military arm-twisting to a coup against a democratically elected president.
The Tamarod, or Rebel, movement which organized the protests has given the president until 5 p.m. Tuesday (1500 GMT) to step down or face even larger demonstrations and possible "complete civil disobedience."
In a highly symbolic move, the crowds have camped out at Cairo's Tahrir quare, the birthplace of the 2011 uprising that toppled Hosni Mubarak. They also have massed outside the president's Ittahdiya palace in the leafy suburb of Heliopolis.
Across town, however, Morsi's backers have hunkered down at their own rally site, vowing to resist any attempts to nullifying his election last year and the rise of Islamist voices in Egypt's political affairs after bring muzzled under Mubarak.