Egypt's opposition has called for mass protests after Islamists claimed voters had backed the draft constitution in the first round of the referendum, BBC News reported.

The National Salvation Front urged people to "take to the streets on Tuesday, December 18 to defend their freedoms, prevent fraud and reject the draft".

The coalition said "irregularities and violations" marred voting on Saturday.

Earlier, President Mohammed Morsi's Muslim Brotherhood said 57% had voted "yes", citing an unofficial tally. The result will not be known until after this Saturday's second round.

The opposition had demanded the referendum be postponed because the constituent assembly had approved the draft charter despite a boycott by liberals, secularists and Christians, who complained it did not adequately protect women, freedom of expression or religion.