* ElBaradei draws comparisons with rigged Mubarak polls
* Islamist denies voting will lack credibility
* Mursi to change dates to soothe Christians (Adds expected date changes, quotes, background)
By Alexander Dziadosz
CAIRO, Feb 23 (Reuters) - An opposition leader called on Saturday for Egyptians to boycott elections due to start in April, saying the process under Islamist President Mohamed Mursi would be "an act of deception".
Islamists, who have won every election since the 2011 overthrow of autocrat Hosni Mubarak, dismissed any suggestion that the parliamentary polls would lack credibility and predicted a strong turnout.
While divisions between the Islamists and opposition remain deep, Mursi appeared ready to soothe anger among the Christian minority over the election schedule. The speaker of Egypt's upper house said Mursi would change the dates to avoid Coptic Christians having to vote during their Easter celebrations.
Mursi called the lower house elections on Thursday, aiming to conclude Egypt's turbulent transition to democracy.
However, liberal politician Mohamed ElBaradei drew comparisons with the last parliamentary polls to be held under Mubarak in 2010, a vote which was widely seen as rigged.
ElBaradei, a former U.N. nuclear agency chief, noted he had called for a boycott in 2010 "to expose sham democracy".
"Today I repeat my call, (I) will not be part of an act of deception," he said on his Twitter account.
The Muslim Brotherhood, which backs Mursi, rejected any call to boycott the voting which has been scheduled in four stages from April 27 to June. Essam Erian, a 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 be monitored by Egyptian and foreign civil society and human rights organisations, he said on his Facebook page, adding that he expected wide participation.
Egyptian elections have been supervised by judges since the revolution. The relatively small numbers of judges have required the drawn-out process, allowing them to oversee voting in different regions on different days.
The opposition says Mursi should not have called the elections until a number of disputes had been settled, chiefly a new constitution produced by an Islamist-dominated assembly which contributed to serious street violence last year.
Mursi's announcement of the dates drew fire from Coptic Christians, who make up about 10 percent of the population, as the schedule would interfere with their Easter festival.
Ahmed Fahmy, the speaker of the Shura Council, said Mursi would respond to these worries. "The president answered the requests of the Coptic members and will issue a statement changing the dates of the elections," he told lawmakers.
The state news agency MENA quoted a presidential aide as saying a statement on new dates would be issued "within hours".
Islamists have used well-organised campaign operations to win every election since the revolution, while the liberal and leftist opposition has been beset by divisions. Previous opposition boycott threats have failed to materialise.
The National Salvation Front (NSF), which groups a number of parties opposed to the Islamists including ElBaradei's, is due to decide in the coming week whether to join a boycott.
"This is Dr. ElBaradei's own position and own opinion," said NSF spokesman Khaled Dawood, but he added that other NSF leaders were sympathetic to the idea of a boycott.
"This is yet another individual move by the Muslim Brotherhood and President Mursi, establishing facts on the ground and then asking you to basically go with the rules of the game they've set on their own," he said. (Writing by David Stamp; Editing by Rosalind Russell)