Egyptian authorities say no date has been set for official results of this month's two-stage referendum on an Islamist-backed draft constitution, as authorities investigate opposition complaints of fraud.

Speaking Monday, members of Egypt's election commission also said they were still compiling results from the staggered voting in Egypt's 27 provinces on December 15 and 22.

The Muslim Brotherhood movement of President Mohamed Morsi said Sunday that unofficial results show the constitution won approval from 64 percent of voters in the two rounds. It said turnout was about 32 percent.

The liberal opposition National Salvation Front filed complaints with the election commission Sunday, alleging that a lack of judicial supervision of the referendum led to rigging and intimidation of voters by Islamists. Many judges boycotted the process to protest Mr. Morsi's recent power grab that briefly put his decisions above the law.

The opposition coalition views the draft constitution as a threat to civil liberties. In a Sunday news conference, coalition members vowed to keep fighting the charter in a peaceful and democratic manner even if the election commission certifies its passage.

Islamists said the referendum was fair and represents a crucial step in Egypt's transition to democracy, almost two years after a popular uprising ousted longtime president Hosni Mubarak.

If approved, the constitution requires the staging of elections for the lower house of parliament within two months. The previous Islamist-dominated assembly was dissolved by Egypt's then-military rulers in June, leaving all legislative powers in the hands of President Morsi, who took office later that month.

Wrangling between liberals and Islamists over the constitution appears likely to continue in the next lower house, potentially prolonging Egypt's political crisis as the country struggles to revive an ailing economy.

Ratings agency Standard and Poor's downgraded its rating on Egypt's long-term sovereign debt from B to B-minus on Monday, blaming the move on the country's political turmoil. It also warned that another rating cut was possible “if a significant worsening of the domestic political situation results in a sharp deterioration of economic indications such as foreign exchange reserves or the government's deficit.”

An Islamist-dominated assembly finalized the draft constitution last month after liberals and Christians walked out, complaining their views were being ignored. Opposition groups object to the charter because it increases the role of Islamic law in society and does not explicitly mention the rights of women or minorities.

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