Nobel laureate Mohammed El-Baradei told about 200 activists gathered Monday for a sunset Ramadan meal that participating in the vote would go against "the national will" to transform Egypt into a genuine democracy.
"If the whole people boycott the elections totally, it will be in my view the end of the regime," he told reporters afterward.
Egypt's opposition groups are divided over the issue of a boycott and it is not clear how many would heed a call not to contest or vote in the election. The largest opposition force, Egypt's Muslim Brotherhood, for example, is unlikely to boycott, although it backs El-Baradei and his demands for change.
El-Baradei, who served as the chief of the UN nuclear agency, returned home in February to a hero's welcome.
Supporters have rallied behind him to press for democratic reforms and urged him to run in the 2011 presidential election.
El-Baradei, whose campaign has provoked government anger, said he would only run if there were constitutional changes and guarantees of free elections. In six months, his campaign has gathered around 800,000 signatures on a petition calling for such changes - a force that seems to have encouraged El-Baradei to attack the government more forcefully.
Until Egypt's political system opens up, it would be wrong to legitimize it by participating in elections, he said. By pressing from the outside, the regime is more likely to give way, he said.
El-Baradei was not specific on where he would take his campaign next, but he threatened civil disobedience if the regime continues to ignore calls for change. However, for the time being, he urged his supporters to reach out for bigger numbers.
The crowd at Monday's event chanted: "El-Baradei keep on going. We are behind you for change. There is no going back."
