Egyptian opposition groups say the first round of a referendum on an Islamist-backed constitution was marred by widespread violations and should be re-staged.

Egypt's main opposition coalition, the National Salvation Front, said Sunday it will not accept any unofficial referendum results. They called for nationwide demonstrations Tuesday before the second round is held next Saturday.

The Muslim Brotherhood, backers of President Mohamed Morsi, claimed victory in Saturday's vote, saying unofficial tallies showed 56 percent approval of the draft constitution.

The Brotherhood said turnout was about one-third of the 26 million people eligible to vote. The first round of the referendum was held in 10 of Egypt's 27 regions, including the two main cities of Cairo and Alexandria. A second round will be held in Egypt's remaining provinces, most of which are rural and religiously conservative.

Some observers expect the second round to boost the overall “yes” vote for the draft constitution, which requires only a simple majority for passage.

Saturday's vote was largely peaceful, with more than 100,000 Egyptian troops deployed to prevent violence. But as the polls closed, some Islamists assaulted the Cairo offices of a liberal newspaper affiliated with the opposition Wafd party.

Opposition activists say they fear the constitution will erode civil liberties because it boosts the role of Islamic law and does not mention women's rights. An Islamist-dominated constituent assembly drafted the document last month after liberal and Christian members walked out, complaining they were being ignored.

Islamist supporters of the draft constitution say its passage in a referendum is an important step toward Egypt's transition to democracy, almost two years after a popular uprising toppled longtime President Hosni Mubarak.

The controversy over the constitution sparked rival mass protests by opposition and Islamist activists earlier this month. At least eight people were killed in street battles among demonstrators.