Egypt's opposition coalition said Sunday it would appeal the outcome of the referendum on a draft constitution, which it alleged was marred by fraud and irregularities. The opposition National Salvation Front also said it was moving toward forming a single political party to challenge Islamists, whose more disciplined ranks have dominated the ballot box since last year's revolution.

The opposition called for an investigation into allegations of vote fraud in the referendum on a deeply divisive Islamist-backed constitution, after the Muslim Brotherhood, the main group backing the charter, claimed it passed with a 64 percent "yes" vote on a turnout of about a third of the 51 million eligible voters.

Official results have not been released yet and are expected on Monday. If the unofficial numbers are confirmed, it will be a victory for the Islamist president and a defeat for liberals, socialists and other factions that united under the banner of the National Salvation Front to campaigned for a "no" vote.

Members of the opposition, whose differences split the non-Islamist vote, pledged to keep up the pressure on Egypt's President Mohamed Morsi, including through peaceful protests.

"We are ready to go through all battles of democracy," said Hamdeen Sabahy, one of the alliance's leaders said. "Our participation in the elections will be a key test for us, and we will not allow fraud to be repeated in the upcoming parliamentary elections."

"The Front is very cohesive and the Front is in agreement that it will lead all battles together," Mohamed Abul Ghar, head of the Egyptian Social Democratic Party and a leading member of the Front, told a news conference after the referendum.

"Not only that, but the parties inside the Front have taken advanced steps to form a big party inside the Front," he said.

A statement from the Front said it had learnt "useful lessons" during the referendum. But it will have little time to organise, as a parliamentary election is due to be held in about two months.

'The revolution continues'

Votes since the overthrow of autocratic leader Hosni Mubarak in February 2011 suggest Islamist support has slipped, but Morsi's Muslim Brotherhood is a potent political force with a grassroots network - built up over decades, even when it was repressed - that liberals cannot yet match.

The opposition says the constitution, passed after weeks of protests and violence, favors Islamists and ignores the rights of Christians, who make up about 10 percent of the population, and women. They say it will split the country and lead to more unrest.

"The majority is not big and the minority is not small," liberal politician Amr Hamzawy said, adding that the Front would use "all peaceful, democratic means" to challenge the constitution and make their voices heard.

George Ishak, another Front figure, said: "The revolution continues and we will resist with all peaceful means to bring down this unjust constitution."

Other members said discussions about fighting the parliamentary poll as a single unit were continuing and it was too early to talk of details such as how candidates would be fielded in different constituencies.

Front members include Nobel Peace Prize winner Mohamed ElBaradei, who leads the Constitution Party, and leftist Hamdeen Sabahy, founder of the Popular Current movement.

Both groups have been prominent in demonstrations against Morsi's rule and his drive to fast-track the constitution through an Islamist-dominated drafting assembly, which opponents quit in protest.

Sabahy said the referendum showed "this constitution has no national consensus." Islamists dismissed such criticism, saying the result was a clear majority and the constitution was a fair and essential step to advance Egypt's democratic transition.