President Mohamed Morsi has brought forward the start of the country's parliamentary elections to April 22 after members of the Coptic Christian minority criticised the planned timing of the polls.
The first round of voting in Cairo and four other provinces had been scheduled for 27 April, which would have seen some voting take place during the Easter holiday.
Opposition politician Mohamed ElBaradei called for a boycott of parliamentary elections, saying he refused to take part in "an act of deception".
"Today I repeat my call, [I] will not be part of an act of deception," ElBaradei, a former UN nuclear agency chief and a Nobel Laureate, said on his Twitter account.
ElBaradei said that he had called in 2010 for a similar boycott of polls held under Mubarak, who was toppled the following year.
The Muslim Brotherhood, which backs Morsi, dismissed suggestions that the elections, to be held in four stages from April to June, would lack credibility.
Essam Erian, senior member of the Brotherhood's Freedom and Justice Party, said the polls would be carried out under "complete judicial supervision" as well as being followed by Egyptian, regional and international media.
Voting would also be monitored by Egyptian and foreign civil society and human rights organisations, he said on his Facebook page, adding that he expected wide participation.
ElBaradei's call appeared to reflect confusion within the National Salvation Front (NSF), which groups a number of parties opposed to the Islamists - including his own Constitution Party.
On Friday NSF spokesman Khaled Dawood said the front would meet in the coming week to decide whether to participate.
Previous opposition boycott threats have failed to materialise.