Egypt's new constitution, drafted by Islamist supporters of President Mohamed Morsi, has been approved by 63.8 percent of voters in a two-round referendum, the supreme election committee said on Tuesday.

The result, which followed votes held on Dec. 15 and on Dec. 22, matched an earlier unofficial tally given by Morsi's Muslim Brotherhood.

"We have seriously investigated all the complaints," judge Samir Abu el-Matti of the Supreme Election Committee told a news conference. The final official turnout was 32.9 percent.

Morsi's critics said the vote, conducted in two stages in a process that ended on Saturday, had been marred by a litany of irregularities, and have demanded a full inquiry.

The opposition, a loose alliance of socialists, liberal-minded Muslims and Christians, have also noted that less than a third of those eligible turned out to vote, undermining the legitimacy of the new constitution.

Highlighting investor concerns, Standard and Poor's cut Egypt's long-term credit rating and said another cut was possible if political turbulence worsened.

Responding to what it said were market rumors, the central bank said it was taking steps to safeguard bank deposits.

Some Egyptians say they have withdrawn their funds from banks out of concern that they will be frozen by authorities.

Legislative powers

Under the new constitution, legislative powers that have been temporarily held by Morsi move to the Islamist-dominated upper house of parliament until a new lower house is elected.

The make-up of the Supreme Constitutional Court, which Islamists say is filled with Mubarak-era appointees bent on throwing up legal challenges to Morsi's rule, will also change as its membership is cut to 11 from 18.

The low turnout in voting on the constitution has prompted some newspapers to question how much support the charter really had, with opponents saying Morsi lost the vote in much of the capital.

"The referendum battle has ended, and the war over the constitution's legitimacy has begun," said newspaper Al-Shorouk, while a headline in Al-Masry Al-Youm read: "Constitution of the minority".

The head of the Muslim Brotherhood's Freedom and Justice Party, Saad al-Katatni, wrote on Facebook that the group's members were "extending our hands to all political parties and all national forces", adding: "We will all start a new page."

But the opposition National Salvation Front says the new constitution only deepens a rift between the liberals and Islamists who combined to overthrow Mubarak, and that they will keep challenging it through protests and other democratic means.

"We do not consider this constitution legitimate," liberal politician Amr Hamzawy said on Sunday, arguing that it violated personal freedoms.

The run-up to the referendum was marred by protests triggered by Mursi's decision to award himself broad powers on Nov. 22. At least ten people were killed in clashes in Cairo and violence also flared in Egypt's second city, Alexandria.