Mohammed ElBaradei, former head of the International Atomic Energy Agency, has been welcomed home by hundreds of people outside Cairo airport, BBC reported.
Mr ElBaradei has not lived in his homeland for decades but is considering running for the presidency in next year's election.
Mr ElBaradei was briefly unable to leave the VIP lounge at the airport after his plane arrived.
Security forces had earlier warned his backers not to gather at the airport.
More than 1,000 people had waited for his arrival, waving banners and chanting support for Mr ElBaradei.
The BBC's Cairo correspondent, Christian Fraser, says that roadblocks were imposed on approaches to the airport and that cameras were banned inside the airport.
Magdi Abdelhadi, the BBC's Arab affairs analyst, says that for some Egyptians Mr ElBaradei's appeal lies in the fact that he is a civilian - Egypt has been ruled by the military since the monarchy was overthrown more than 50 years ago.
He has also been untainted by corruption allegations.
But detractors, writing in the state media, say that he is a figure who is out of touch with Egyptian life, having lived abroad for so many years.
Mr ElBaradei, 67, has built a strong reputation as head of the International Atomic Energy Agency (IAEA), winning the Nobel Peace Prize in 2005.
He stepped down in November and is now seen as the most credible potential challenger for the presidency.
He has said he might stand if there were reforms to guarantee a fair election and if he could run as an independent candidate. But for this to happen the constitution would have to be amended.
President Hosni Mubarak, 81, has ruled Egypt for nearly 30 years and there is much speculation he is grooming his son Gamal to take over when he steps down.
Opposition and civil society groups have long complained the authorities have used emergency laws and the security forces to curb political freedoms.
The largest opposition party, the religious Muslim Brotherhood, is banned and its candidates have to stand as independents.
The campaign on the social networking website Facebook has become a focal point for those calling for Mr ElBaradei to run for the presidency.
