Mr. Obama watched Mr. Mubarak's speech on board Air Force One, returning from a trip to Michigan, the press secretary, Robert Gibbs, said. As soon as he arrived at the White House, Mr. Obama huddled with his national security aides. The administration appeared as taken aback by Mr. Mubarak's speech as the crowds in Tahrir Square. The director of the Central Intelligence Agency, Leon E. Panetta, testified before the House of Representatives on Thursday morning that there was a "strong likelihood" that Mr. Mubarak would step down by the end of the day.

American officials said Mr. Panetta was basing his statement not on secret intelligence but on media broadcasts, which began circulating before he sat down before the House Intelligence Committee. But a senior administration official said Mr. Obama had also expected that Egypt was on the cusp of dramatic change. Speaking at Northern Michigan University in Marquette, he said, "We are witnessing history unfold," adding, "America will do everything we can to support an orderly and genuine transition to democracy."