* Opposition coalition takes 'advanced steps' towards single party
* Members vow to keep up resistance to new constitution
* Challenge to organise for election in two months
By Shaimaa Fayed
CAIRO, Dec 23 Egypt's opposition coalition said on Sunday it was moving towards forming a single political party to challenge Islamists, whose more disciplined ranks have dominated the ballot box since last year's revolution.
Members of the opposition National Salvation Front, whose differences have split the non-Islamist vote, pledged to keep up the pressure on President Mohamed Mursi, including through peaceful protests.
Liberals, socialists and other factions that united under the banner of the Front campaigned unsuccessfully for a "no" vote in a referendum on a new constitution which, according to an unofficial tally by Mursi's Islamist backers on Sunday, secured 64 percent approval on turnout of about a third of the 51 million eligible voters.
"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 Mursi'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, favours 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 Mursi'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.