President Barack Obama has declared that the upheaval in Egypt has “changed the world” and pledged US support for a “genuine” transition to democracy.

US officials welcomed President Hosni Mubarak’s departure from power – which they had expected a day earlier – but scrambled to get up to speed with the Egyptian military’s assumption of power. Some officials had expected Omar Suleiman, the vice-president, to take control.

In remarks that proclaimed the Egyptian people had “changed their country and in doing so changed the world”, Mr Obama was careful to call on the military to lift Egypt’s emergency law and revise the constitution to make change irreversible.

Describing the events as only the beginning of the country’s transition, he added the US would “stand ready to deliver whatever assistance is necessary” to make the country a “genuine democracy”.

Officials expressed hope that Egypt would chart a middle course between a rupture with the country’s constitution and a narrow reading that would have empowered the speaker of the country’s parliament.

“We believe that it is possible under the constitution for the kinds of changes that are needed to hold free, fair and credible elections,” said an administration official.

The Egyptian army’s decision to intervene came after extensive contact with the Pentagon. The US has long cultivated the Egyptian army, not least through $1.3bn in annual aid.

On Thursday, Robert Gates, defence secretary, and Admiral Mike Mullen, chairman of the joint chiefs of staff, called their counterparts, the now newly empowered Field Marshal Mohammed Hussein Tantawi, defence minister, and Sami Enan, army chief of staff. In Mr Gates’s case, it was the fifth such call since the crisis began.

The US has welcomed the army’s intervention while warning that it must not carry out a coup but instead usher in real democratic rule. Officials from Mr Obama down insist that the new government must still prove its commitment to democratic reform.

Joe Biden, vice-president, said: “The transition that is taking place must be an irreversible change and a negotiated path to democracy.”

Reflecting concerns at how the upheaval might affect relations in the region, Robert Gibbs, Mr Obama’s spokesman, said it was important the new government respected peace accords with Israel.

But he sidestepped questions as to whether the US was calling for similar democratic uprisings in allies such as Jordan and Saudi Arabia.

Mr Gibbs acknowledged the US had been expecting a “different speech” on Thursday when in an address to the nation, Mr Mubarak refused to resign, in spite of mounting expectations to the contrary.

In statements that gave the impression that the US was out of touch that day, Leon Panetta, CIA director, had said there was a “strong likelihood” Mr Mubarak would step down by the evening. Mr Obama himself even announced that the world was “witnessing history”.

On Friday, the president repeated the phrase. This time, however, Mr Mubarak was safely out of office.