Egypt's liberal opposition coalition says it will challenge the outcome of a referendum on an Islamist-backed constitution that appears to have won majority approval. The two-stage vote was marred by low turnout and accusations of widespread fraud.

Speaking Sunday, members of the opposition National Salvation Front said they plan to file complaints about voting irregularities after the official referendum results are announced, likely on Monday. The two-stage process ended Saturday with voting in 17 of Egypt's 27 provinces. It began with voting in the other 10 provinces on December 15.

The Muslim Brotherhood movement of Islamist President Mohamed Morsi said Sunday that unofficial results show the constitution won 64 percent approval over the two rounds of voting. It said Saturday's “Yes” vote was even higher, at 71 percent. The Islamist group has accurately predicted the outcomes of previous elections.

The Brotherhood also said the referendum's combined voter turnout was 32 percent, a figure cited by opposition activists as showing the lack of a popular mandate for the constitution, which was drafted by an Islamist-dominated assembly.

Islamist and opposition groups had staged a series of mass protests for and against the constitution in recent weeks. Some members of the rival groups engaged in violent street battles that killed eight people outside the presidential palace in Cairo.

If Egypt's electoral commission confirms the passage of the referendum, an election would be held for a new lower house of parliament in two months. It is not clear if the opposition coalition will continue to call for mass protests against the constitution or focus on campaigning to try to amend the document from within parliament.

The Muslim Brotherhood has promoted the constitution as a crucial step in Egypt's transition to democracy, almost two years after a popular uprising ousted longtime president Hosni Mubarak. But, Egypt's liberal, secular and Christian opposition groups fear the charter will erode civil liberties, because it increases the role of Islamic law in society and does not explicitly mention women's rights.