Mohammed ElBaradei told protesters Sunday in Tahrir Square in central Cairo that Egypt was witnessing an "historic day" and that the process of change would move forward, dpa reported.
"What we have begun today cannot go back," the opposition figure said.
Broadcaster al-Jazeera quoted ElBaradei as saying "we are beginning a new era in Egypt."
Tens of thousands are gathered in Cairo in the sixth day of anti- government protests, defying a curfew. Other protests are taking place in cities across the country.
But some of those in Tahrir Square were split over the opposition figure.
Many in the liberal camp see the Nobel Peace Prize winner as a leader for their country who has the ear of the West and can push through democratic and economic reforms - but the view is not universal.
"We are disappointed in ElBaradei, we expected more of him, to better organise the opposition," said medical student Abdullah Sahrani. He would be accepted as a temporary leader but "he has to prove himself" as capable of guiding the country.
"I would not accept him to be president because he is not of this country, but he could take temporary control," said another protester, referring to ElBaradei's many years abroad, including time at the helm of the UN's nuclear watchdog.
Others felt only a military man could take the helm of Egypt, a poor country of 80 million people.
"I reject ElBaradei as a leader. The leader must come from the armed forces," said Abu Bakr, a worker from Cairo.