CAIRO - A coalition of the Muslim Brotherhood and other Islamist groups called on Saturday for a referendum on a draft constitution to go ahead on time on December 15, after liberals who complain that the document is biased demanded a delay in the vote.The main Islamist coalition "confirmed the need for measures to hold the referendum on time on December 15 without any amendment or delay", a spokesman for the group told a news conference. Among those who spoke at the news conference, aired live on television, was Khairat al-Shater, who had been the Brotherhood's candidate for president until he was disqualified, a move that thrust Mohamed Morsi into his place and into office.Meanwhile, Egypt's military on Saturday urged rival political forces to solve their disputes via dialogue and said the opposite would drag the country into a "dark tunnel", which it would not allow.A statement issued by the military spokesman and read on state radio and television made no mention of President Mohamed Morsi, but said a solution to the political crisis should not contradict "legitimacy and the rules of democracy".The spokesman said the military's duty was to protect national interests and secure vital state institutions."The armed forces ... realize their responsibility to preserve the higher interests of the country and to secure and protect vital targets, public institutions and the interests of innocent citizens," the statement said."The armed forces affirm that dialogue is the best and only way to reach consensus," it added. "The opposite of that will bring us to a dark tunnel that will result in catastrophe and that is something we will not allow."The statement said Egyptians were capable of expressing their views peacefully "far from all displays of violence."'Morsi may delay on constitution referendum'Vice President Mahmud Mekki told AFP on Friday that Morsi "could accept to delay the referendum" on a draft constitution disputed by the opposition, .Mekki said the president could delay the 15 December referendum if the opposition guaranteed not to challenge the move on those grounds later, AFP news agency reported. Morsi was expected to press ahead on Saturday with talks on ways to end Egypt's worst crisis since he took office even though the country's main opposition leaders have vowed to stay away.Cairo and other cities have been rocked by violent protests since November 22, when Morsi promulgated a decree awarding himself sweeping powers that put him above the law.The upheaval in the most populous Arab nation, following the fall of Hosni Mubarak last year, worries the West, in particular the United States, which has given it billions of dollars in military and other aid since Egypt made peace with Israel in 1979.Morsi's deputy raised the possibility that a referendum set for December 15 on a new constitution opposed by liberals might be delayed. But the concession only goes part-way towards meeting the demands of the opposition, who also want Morsi to scrap the decree awarding himself wide powers.On Friday, large crowds of protesters surged around the presidential palace, breaking through barbed wire barricades and climbing on tanks guarding the seat of Egypt's first freely elected president, who took office in June.As the night wore on, tens of thousands of opposition supporters were still at the palace, waving flags and urging Morsi to "Leave, leave".