CAIRO — A defiant President Hosni Mubarak refused to step down Thursday, saying in a televised speech that he'd remain Egypt's head of state until elections in the fall but had delegated powers to his vice president.

Anticipation turned to rage in Cairo's main square, where hundreds of thousands of Egyptians had gathered in hopes of witnessing history: a 17-day-old popular rebellion ending 30 years of U.S.-supported authoritarian rule. When Mubarak made it clear that he would stay, protesters sobbed, embraced in anguish and chanted, "Leave! Leave!"

The brief speech, Mubarak's third since the revolt began on Jan. 25, revealed an octogenarian president who appears willing to risk an escalation of violence before giving up his post. To those allied with the protest movement, Mubarak's refusal to step aside amounted to a declaration of war against his own people, and they vowed retaliation in nationwide demonstrations planned for Friday.

Late Thursday, satellite TV channels reported that hundreds of protesters had gathered in front the heavily guarded presidential palace and were planning marches on other sensitive sites, an ominous sign for the rallies Friday.

The crowds in Tahrir Square booed and showed their disgust by waving their shoes when Mubarak, in tones the protesters described as patronizing, waxed nostalgic about his military service and affirmed his commitment to reforming the constitution, which was designed to keep his regime in power.

The constitution, Mubarak said, allowed him to transfer executive power to his newly appointed vice president, former intelligence chief Omar Suleiman, who already had been tasked with overseeing the reform process. The constitution allows such a move "due to any temporary obstacle" but doesn't require the president's resignation, nor does it bar him from reclaiming the powers at a later date.

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In a strongly worded statement Thursday evening, President Barack Obama urged Egyptian officials to "explain the changes . . . and to spell out in clear and unambiguous language" the steps that the regime is taking to address protesters' demands.

"The Egyptian people have been told that there was a transition of authority, but it is not yet clear that this transition is immediate, meaningful or sufficient," Obama said. "The Egyptian government must put forward a credible, concrete and unequivocal path toward genuine democracy, and they have not yet seized that opportunity."

Stephen P. Cohen, a U.S. scholar who's served as an intermediary between Arab and Israeli leaders and has known Suleiman for years, said Mubarak's refusal to step down is pushing Egypt into "the kind of dangers of an endgame" of unknown consequences.

"People are behaving in ways that are not entirely rational right now," said Cohen, the head of the Institute for Middle East Peace and Development in New York. "But I don't think the military is going to change its mind about crushing the demonstrators. I don't think it will do that."

He said it's unlikely that Suleiman will be any more successful at resolving the crisis with the executive powers conferred on him by Mubarak than the Egyptian leader himself has been.

"I think that he (Mubarak) has basically given Suleiman damaged goods because he no longer has much credibility because of the person who is giving him the appointment, and because of the way they are alienating the people," Cohen said.

Mubarak also said panels of independent legal experts were making progress on constitutional amendments that would ease conditions for presidential candidacy, set term limits and offer more judicial oversight in elections — all issues that were among the protesters' chief demands.

"I have given you this vision under commitment to take the country out of this current crisis and I will continue to accomplish it and monitor the situation hour by hour," Mubarak said. "I'm looking forward to the support of all those who are concerned about security and want a secure Egypt."

All eyes are now on Egypt's army, perhaps the last credible institution in the country, which had served as a buffer between the regime and the protesters. Opposition leaders said the military should make clear its position: Would it continue to protect the foundering government, or side with protesters who've had their fill of hollow promises?

"Egypt will explode. Army must save the country now," the Nobel laureate and opposition figure Mohamed ElBaradei wrote on his Twitter account.

All day, rumors had swirled that Mubarak would step down and hand over authority to a military-led council. Expectations soared when the military announced on state television that its supreme council was in session, an indication that it was prepared to take control. The military had no immediate reaction after the speech.

"It's becoming clearer that run-of-the-mill negotiations aren't going to force him to leave," said a U.S. official, who asked not to be further identified because of the sensitivity of the issue. "He appears to be gambling that the army is going to stay loyal." Mubarak is "operating in somewhat of a vacuum, I would suspect," added the official.

(Jonathan S. Landay contributed from Washington. McClatchy special correspondent Miret el Naggar contributed from Cairo.)

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