CAIRO >> The fate of Egypt’s first democratically elected president hung in the balance today, hours before a deadline to yield to the demands of millions of protesters or see the military suspend the constitution, disband parliament and install a new leadership.

Embattled Islamist President Mohammed Morsi vowed not to resign, however, and he demanded that the powerful armed forces withdraw their ultimatum, saying he rejected all "dictates" — from home or abroad.

In a speech to the nation, he pledged to protect his "constitutional legitimacy" with his life and accused loyalists of his autocratic predecessor Hosni Mubarak of riding the current wave of protests to topple his regime.

"There is no substitute for legitimacy," said Morsi, who at times angrily raised his voice, thrust his fist in the air and pounded the podium. He warned that electoral and constitutional legitimacy "is the only guarantee against violence."

Morsi’s defiant statement sets up a major confrontation between his Islamist supporters and Egyptians angry over what they see as his efforts to impose control by his Muslim Brotherhood as well as his failure to introduce reforms more than two years after the Arab Spring revolution. His opponents say that he has lost his legitimacy through mistakes and power grabs and that their turnout on the streets shows the nation has turned against him.

Millions of jubilant, chanting Morsi opponents filled Cairo’s historic Tahrir Square, as well as avenues adjacent to two presidential palaces in the capital, and main squares in cities nationwide. After Morsi’s speech, they erupted in indignation, banging metal fences to raise a din, some raising their shoes in the air in a show of contempt. "Leave, leave," they chanted.

Morsi "doesn’t understand. He will take us toward bloodshed and civil war," said Islam Musbah, a 28-year-old protester sitting on the sidewalk outside the Ittihadiya palace, dejectedly resting his head on his hand.

The president’s supporters also increased their presence in the streets of the capital and other cities, after the Muslim Brotherhood and hard-line Islamist leaders called them out to defend what they say is the legitimacy of his administration.

At least seven people were killed in three separate clashes between his supporters and opponents in Cairo, according to hospital and security officials who spoke on condition of anonymity because they were not authorized to talk to the media. At least 23 people have died in political violence since the unrest began on Sunday, the first anniversary of Morsi’s inauguration.

Morsi’s supporters have stepped up warnings that it will take bloodshed to dislodge him, saying they would rather die fighting a military takeover than accept his ouster just a year after Egypt’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-Beltagy wrote today in his official Facebook page.

Monday, the military gave Morsi an ultimatum to meet the protesters’ demands within 48 hours. If not, the generals’ plan would suspend the Islamist-backed constitution, dissolve the Islamist-dominated legislature and set up an interim administration headed by the country’s chief justice, the state news agency reported.

The leaking of the military’s so-called political "road map" appeared aimed at adding pressure on Morsi by showing the public and the international community that the military has a plan that does not involve a coup.

On his official Twitter account, Morsi urged the armed forces "to withdraw their ultimatum" and said he rejects any domestic or foreign dictates."

Fearing that Washington’s most important Arab ally would descend into chaos, U.S. officials said they are urging Morsi to take immediate steps to address opposition grievances, telling the protesters to remain peaceful and reminding the army that a coup could have consequences for the massive American military aid package it receives. The officials spoke on condition of anonymity because they were not authorized to speak publicly.

Morsi adviser Ayman Ali denied that the U.S. asked Egypt to call early presidential elections and said consultations were continuing to reach national conciliation and resolve the crisis. He did not elaborate.

Morsi met with the army’s chief, Defense Minister Gen. Abdel-Fattah el-Sissi, and Prime Minister Hesham Kandil in the second such meeting in as many days, Ali said, without giving details.

The army has insisted it has no intention to take power. But the reported road map showed it was ready to replace Morsi and make a sweeping change in the ramshackle political structure that has evolved since Mubarak’s fall in February 2011.

The constitution and domination of the legislature after elections held in late 2011-early 2012 are two of the Islamists’ and Brotherhood’s most valued victories — along with Morsi’s election last year.

A retired army general with close ties to the military confirmed the news agency report’s version of the road map.

Hossam Sweilam said a panel of experts would draft a new constitution and the interim administration would be a presidential council led by the Supreme Constitutional Court’s chief justice and including the defense minister, representatives of political parties, youth groups, Al-Azhar Mosque and the Coptic Church.

He said the military envisioned a one-year transitional period before presidential elections are held.

The military spokesman, Col. Ahmed Mohammed Ali, declined to confirm the details. "It is too early and we don’t want to jump into conclusions," he said.

At least one anti-Morsi TV station put up a clock counting down to the end of the military’s ultimatum, putting it at 4 p.m. Wednesday (1400 GMT, 10 a.m. EDT), though a countdown clock posted online by Morsi opponents put the deadline at 5 p.m. (1500 GMT, 11 a.m. EDT). The military did not give a precise hour.

Morsi also faced new fissures within his leadership.

Three government spokesmen — two for Morsi and one for the prime minister — were the latest to quit as part of high-level defections that underscored his increasing isolation and fallout from the military’s ultimatum. Five Cabinet ministers, including the foreign minister, resigned Monday, and a sixth, Sports Minister El-Amry Farouq, quit today.

One ultraconservative Salafi party, al-Nour, also announced its backing for early elections. The party was once an ally of Morsi but in recent months has broken with him.

Among the opposition crowds outside Qasr el-Qobba, one protester said he believes Morsi will not go easily. "He will only leave after a catastrophe. Lots of blood. And the military is the only party that can force him out then," said Haitham Farouk, an oil company employee joining a protest for the first time.

He said the "epic" crowds showed how Egypt’s public has turned against Morsi and his Brotherhood, which opponents claim is the real power behind the president. "This is everybody, not just the educated or the political," Farouk said of the protesters. "They came down because only the Brotherhood gained in the past two years.

Morsi may try half-measures to satisfy the army, he said, "but the people are not going back until he leaves. After what we have seen in the past year, we will not settle for less."

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 move appeared aimed at presenting a unified voice in a post-Morsi system, given the widespread criticism that the opposition has been too fragmented to present an alternative to the Islamists.

Brotherhood spokesman Gehad el-Haddad said the opposition is to blame for its own woes, failing to perform well in the elections, and has now decided to "brush up to military power."

"We can’t keep running elections until (the Brotherhood) loses," he wrote in a Tweet. He said the opposition should "man up" to its responsibilities and come up with a better strategy "or accept democratic outcomes."

Despite heated rhetoric among many Islamists about standing up to the military, one cleric from the Salafi movement warned against repeating the scenario of Algeria, when the military negated elections that Islamists won in the 1990s, and the Islamists responded with a yearslong, bloody insurgency.

The result, Adel Nasr wrote on a Salafi website, was that "more than a hundred thousand were killed and … their popularity went down," costing Islamists both political power and the power of their religious message.