Many Egyptians have given Mohamed ElBaradei a warm welcome home
Former UN nuclear chief Mohamed ElBaradei has used his Facebook web page to call for reform in Egypt, where he is now spearheading the opposition.
He used his video message to urge all Egyptians to join his newly formed group, National Association for Change.
Mr ElBaradei has hinted he may stand against 81-year-old President Hosni Mubarak, who has ruled since 1981, in an election due in 2011.
Observers believe Mr Mubarak wants his son Gamal to succeed him.
The BBC's Christian Fraser in Cairo says Mr ElBaradei's appearance on Facebook was clearly designed to shake the Egyptian opposition out of its slumber.
In his latest message Mr ElBaradei says it is the responsibility of the individual to opt for change.
And he urged his supporters to send a peaceful message to the ruling establishment that it is time for reform.
The Facebook social networking website has become a focal point for those calling on him to stand against Mr Mubarak.
Our correspondent says there is growing momentum behind the campaign and Mr ElBaradei, who returned from decades living abroad last month, appears to be warming to the challenge.
But the state tolerates little dissent and has already fought back - arresting one activist who was running a Facebook group supporting Mr ElBaradei, our correspondent adds.
Analysts say Mr ElBaradei's appeal for many Egyptians is that he is a civilian in a country long ruled by soldiers, and that he is untainted by corruption allegations.
But detractors writing in state media have portrayed him as a figure who is out of touch with Egyptian life, having lived abroad for so many years.
Mr ElBaradei, 67, built a strong reputation as head of the International Atomic Energy Agency (IAEA) and won 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.