|Obama said that we had witnessed history in Egypt over the past 18 days and that by stepping down Hosni Mubarak responded to the Egyptian people's hunger for change.
The US president stressed, however, that Mubarak's exit was not an end, but rather a beginning.
He expressed confidence that the people of Egypt could continue there transition to democracy peacefully and with the same sense of unity they have displayed through their protest.
Obama said that he Egyptian military had served patriotically and responsibly as a caretaker of the state and must ensure a transition to a democracy that is "irreversible" and credible. He stressed that the formation of the new government must allow for all of Egypt's voices to be heard.
Earlier on Friday, US Vice President Joe Biden responded to Egyptian President Hosni Mubarak's resignation saying, "It is a historic day for the people of Egypt."
The development came just one day after Mubarak had declared that he was not resigning, despite all signs to the contrary, which enraged the protesting masses and had a dismayed White House scrambling to respond. Obama had issued a statement Thursday evening in Washington in which he challenged Mubarak, without directly naming him, to explain his actions and his plans for democracy.
And then events changed again.
Obama quickly made plans to speak Friday afternoon from the White House as throngs of activists rejoiced in Cairo.
Lost in the jubilation were questions of who would run Egypt next, and whether the United States would emerge with the kind of stable partner it badly needs in the volatile Middle East.
Still, US lawmakers welcomed Mubarak's resignation.
"I am pleased that President Mubarak has heard and heeded the voice of the Egyptian people, who have called for change," said the leader of the Senate's Democratic majority, Harry Reid of Nevada. "It is crucial that Mubarak's departure be an orderly one and that it leads to true democracy for Egypt, including free, fair and open elections."
On Twitter, Nancy Pelosi, the House Democratic leader, said the young people of Egypt were leading the country to democracy.
"Their actions are an inspiration to the world," she said.
Biden said that throughout the unrest in Egypt, which led to Mubarak's ouster in under three weeks, Republicans and Democrats in the US government have largely spoken with one voice.
"This unity has been important," Biden said. "And it will be even more important in these delicate and fateful days ahead."
Enormous questions loom about how the country will transition to free elections in September, which in turn will affect the important relationship between the United States and Egypt.
The tone from the White House has shifted right along with events. On Thursday afternoon, when Mubarak had been widely expected to step down, Obama was upbeat. "What is absolutely clear is that we are witnessing history unfold," Obama said at the start of an overshadowed economic event in Michigan. "It's a moment of transformation that's taking place because the people of Egypt are calling for change."
Instead Mubarak seemed to dig in defiantly, speaking of ceding power to his vice president and making interim concessions. Obama responded that it was not clear whether that move was a sufficient sign of reform and he called for a "credible, concrete and unequivocal path toward genuine democracy." A day later, Mubarak resigned after all.