FIGHTING

Fighting between Mursi supporters and opponents broke out in the Cairo suburb of Giza, in Alexandria and in the town of Qalyubia, north of Cairo, security sources said.

Protesters thonged Cairo's Tahrir Square, where hovering military helicopters got loud cheers. A quarter of a million packed the square after work, celebrating wildly what they believe is Mursi's impending departure.

Senior Muslim Brotherhood leaders branded the military ultimatum a "coup", backed by a threat that the generals will otherwise impose their own road map for the nation.

The Brotherhood's political wing called for mass counter- demonstrations to "defend constitutional legitimacy and express their refusal of any coup", raising fears of violence. But the biggest pro-Mursi rally in the a Cairo suburb appeared to attract around 100,000 supporters, Reuters witnesses said.

The Brotherhood long avoided direct confrontation with the security forces despite suffering oppression under Mubarak.

After millions protested on Sunday, Sisi delighted Mursi's opponents by effectively ordering the president to heed the demands of the street. It took the president's office nine hours to respond with a statement indicating he would go his own way.

"The president of the republic was not consulted about the statement issued by the armed forces," it said. "The presidency confirms that it is going forward on its previously plotted path to promote comprehensive national reconciliation ... regardless of any statements that deepen divisions between citizens."

Describing civilian rule as a great gain from the revolution of 2011, Mursi said he would not let the clock be turned back.

The United States, which has previously defended Mursi's legitimacy as a democratically elected leader, stepped up pressure on him to heed the mass protests but stopped short of saying he should step down.

President Barack Obama told Mursi in a phone call late on Monday that the political crisis could only be solved by talks with his opponents, the White House said. Secretary of State John Kerry hammered home the message in a call to his outgoing Egyptian colleague on Tuesday.

At least six ministers who are not Brotherhood members have tendered their resignations since Sunday, including Foreign Minister Mohamed Kamel Amr. The president's two spokesmen and the cabinet spokesman also quit on Tuesday and nearly 150 Egyptian diplomats signed a petition urging Mursi to go.

Senior Brotherhood politician Mohamed El-Beltagy denounced what he called a creeping coup. He said he expected the High Committee for Elections to meet within hours to consider annulling the 2012 presidential election.

CONCERN

The United States has long funded the Egyptian army as a key component in the security of Washington's ally Israel.

General Martin Dempsey, chairman of the U.S. Joint Chiefs of Staff, spoke to his Egyptian counterpart on Monday. It is unclear how far the military has informed, or coordinated with, its U.S. sponsors but an Egyptian official said a coup could not succeed without U.S. approval.

The United Nations Human Rights office called on Mursi to listen to the demands of the people and engage in a "serious national dialogue" but also said: "Nothing should be done that would undermine democratic processes."

A senior European diplomat said that if the army were to remove the elected president, the international community would have no alternative but to condemn it.

Yasser El-Shimy, Egypt analyst at the International Crisis Group, said the army ultimatum had hardened positions, making it very difficult to find a constitutional way out of the crisis.

"Things could deteriorate very rapidly from there, either through confrontations on the street, or international sanctions," he said.

"Mursi is calling their bluff, saying to them, 'if you are going to do this, you will have to do it over my dead body'."

For many Egyptians, fixing the economy is key. Unrest since Mubarak fell has decimated tourism and investment and state finances are in poor shape, drained by extensive subsidies for food and fuel and struggling to provide regular supplies.

The Cairo bourse, reopening after a holiday, shot up nearly 5 percent after the army's move.