Egyptian opposition groups are holding last-ditch protests against a referendum on a draft constitution, due to start on Saturday.At least two anti-constitution rallies are to take place in Cairo after Friday's main weekly Muslim prayers.Critics say the draft document, backed by President Mohammed Morsi and his supporters, is too Islamist.Pro-Morsi groups are lining main roads in Cairo, holding placards that say "Yes to the constitution".They have also published online videos with a campaign song that says "This constitution is not too bad, it was written by a committee of heroes", AFP news agency reports.The opposition National Salvation Front has urged a "No" vote but so far stopped short of calling for a boycott. Tight securityThe controversy has sparked bloody clashes across Egypt between President Morsi's supporters and opposition activists.Security is tight, with tens of thousands of police and troops deployed to keep order.President Morsi has also given the army the power to arrest civilians, raising fears that Egypt is moving back towards military rule.A new Egyptian constitution is seen as a major milestone following the revolution that ousted Hosni Mubarak in 2011.But a vote on the draft has sharpened the polarisation between liberal groups and the powerful Muslim Brotherhood that backs Mr Morsi.The president himself has also been at loggerheads with the judiciary.Lack of judgesPolls will open at 08:00 (06:00 GMT) on Saturday in Cairo, Alexandria and eight other provinces. The other half of the country will vote a week later.Polling had to be spread out because so few judges were willing to supervise the vote.Mohamed ElBaradei, a Nobel Peace Prize winner who heads the National Salvation Front, wrote on his Twitter feed: "Insistence on referendum in an explosive, polarised, chaotic and lawless environment is leading country to the brink."Earlier this week, leading opposition figure Hamdeen Sabbahi said the Front would still call for a boycott if key conditions were not met by Saturday.On Friday it was unclear if a boycott would be called.Last weekend, President Morsi offered a concession to the opposition by annulling a 22 November decree that gave him sweeping new powers.