Riot police cordon off all access to a road in Cairo's Doqqi neighbourhood on December 16, 2012
Egypt's opposition is calling for mass protests on Tuesday after Islamists backing President Mohamed Morsi claimed victory in the first round of a referendum it alleges was riddled with polling violations.
The opposition coalition, the National Salvation Front, urged Egyptians to "take to the streets on Tuesday to defend their freedoms, prevent fraud and reject the draft constitution" ahead of the next round of voting on Saturday.
It claimed "irregularities and violations" marred the initial stage of the referendum last weekend across half of Egypt that Morsi's Muslim Brotherhood said resulted in a 57 percent "yes" vote, according to its unofficial tally.
The official count will be given after the other half of the country goes to the polls in the second round.
Large protests both for and against the proposed constitution have been staged over the past three weeks, sparking several violent clashes and revealing deep divisions in Egyptian society over Morsi's rule.
Early this month, eight people died and more than 600 were hurt when rival protesters fought outside the presidential palace in Cairo, prompting the army to deploy troops and tanks to protect it.
Some 250,000 soldiers and police have been mobilised to ensure security during the two-stage referendum.
The opposition Front says the constitution weakens human rights, especially those of women, and undermines the independence of judges while strengthening the hand of the military.
It fears Islamists propelled into power after a revolution last year that toppled the 30-year regime of Hosni Mubarak want to establish sharia-style laws.
Morsi, though, argues that the slender majority support he won in June presidential elections gives him a mandate for change and that the draft constitution is a key step to securing stability.
Many analysts disagree, believing that the lack of consensus is dragging Egypt into a prolonged political conflict.
"Between an Islamist democratic majoritarianism with increasingly authoritarian tendencies and an embittered secular camp tempted by liberal authoritarianism, Egypt's transition is likely to only get more difficult," Issandr El Amrani of the European Council on Foreign Relations wrote in an analysis.
The opposition claims that Saturday's first round of the referendum, which took place in the biggest cities of Cairo and Alexandria and in eight other regions, had numerous violations.
Those included monitors not being allowed into some polling stations, judges not present in all as required and some fake judges employed, and women prevented in some cases from casting their ballot.
Several Egyptian human rights and monitoring groups said on Sunday that the irregularities meant the first round must be held again.
The electoral commission, "in the interest of national consensus," must "recognise that it was not capable (of ensuring) good organisation and it must redo the referendum," said Negad el-Borei, a spokesman for one of the groups, which represents lawyers.