Egypt's change of leaders has to start immediately, President Barack Obama said only hours after Hosni Mubarak promised to quit - but not until elections next September.
"An orderly transition must be meaningful, it must be peaceful, and it must begin now," Mr. Obama said, adding that he had delivered the same message in a telephone call with the 82-year-old Egyptian strongman who has been America's most reliable and long-standing ally in the Middle East for 30 years.
Mr. Obama paid tribute to the Egyptian army for its restraint. He also praised "the passion and the dignity that has been demonstrated by the people of Egypt," saying it was an inspiration to others. The brief remarks will reverberate throughout the region, where pro-democracy uprisings are flaring from Yemen to Sudan to Libya and repressive regimes are suddenly being confronted with popular discontent.
"To the people of Egypt, particularly the young people of Egypt, I want to be clear: We hear your voices," the President said. "I have an unyielding belief that you will determine your own destiny, and seize the promise of a better future for your children and your grandchildren."
The President called the stunning events of the past week, culminating in hundreds of thousands of Egyptians demanding the immediate ouster of the only leader many of them have ever known, "the beginning of a new chapter in the history of a great country."
After days of stronger and stronger hints that Mr. Mubarak must go, Mr. Obama sent retired U.S. diplomat Frank Wisner as a personal envoy Tuesday, who apparently told the Egyptian President the American support was over.
Mr. Obama's carefully crafted remarks delivered across the nation and around the world indicated that Mr. Mubarak may have only half-heeded the American message when he insisted he would serve until new elections.
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That infuriated many Egyptians and - apparently - wasn't sufficient for Mr. Obama.
The American President said he called his Egyptian counterpart. "I spoke directly to President Mubarak [and]he recognizes that the status quo is not sustainable and that a change must take place."
Whether Mr. Obama will openly side with determined anti-Mubarak throngs unwilling to be placated with a promise of graceful departure months hence - or intimidated by a veiled threat of a crackdown on the lawless - will be keenly watched throughout the region.
Other Arab leaders that have been loyal American allies but are now facing unprecedented public opposition will closely watch the still-unfolding drama in Cairo.
"All of us who are privileged to serve in positions of political power do so at the will of our people," Mr. Obama said. "Through thousands of years, Egypt has known many moments of transformation. The voices of the Egyptian people tell us that this is one of those moments, this is one of those times."
Mr. Obama huddled with his national security team after Mr. Mubarak's rambling, unrepentant, late-night speech in Cairo. For days, senior Obama administration officials had been pushing for the Egyptian leader to step down immediately or, at least, set a very short timetable for a handover to a caretaker administration.
"It is not the role of any other country to determine Egypt's leaders," Mr. Obama said. Although he has been careful not to interfere in picking favourites, he is now been very clear that Mr. Mubarak needs to go sooner than the end of summer.