CAIRO (AP) — Egypt's Muslim Brotherhood claimed Sunday that the Islamist-backed constitution has passed with a 64 percent "yes" vote, the day after the final voting in a two-round referendum that deeply divided the country.

The constitution's critics however may contest the outcome. A spokesman for the main opposition group which has been campaigning for a "no" vote said there were "a lot" of irregularities in the voting.

The Brotherhood's unofficial results come a day before the election commission is expected to announce the final official tally for voting organized over two weeks. The group has accurately tallied the outcome of past elections.

The passage of the constitution would be a victory for Islamist President Mohammed Morsi. The Freedom and Justice Party, the Brotherhood's political arm, said in a statement that it hoped the passage is a "historic opportunity" to heal Egypt's divisions and launch a dialogue to restore stability and build state institutions.

But the comparatively low turnout of 32 percent of eligible voters, as well as allegations by the opposition of voting violations, threatened to undermine the constitution's legitimacy and keep Egypt polarized.

Aside from a vocal opposition, Morsi is also facing a fragile economy, weathered by nearly two years of political turmoil and accompanying violence as well as nearly a month of political crisis that preceded the vote.

According to the Brotherhood tally based on results from individual polling stations as well as voting abroad, around 64 percent of the 16.6 million voters who cast ballots approved the constitution.

Saturday's voting in 17 of Egypt's 27 provinces was the second and final round of the referendum. Preliminary results released early by Morsi's Muslim Brotherhood showed that 71 percent of those who voted Saturday said "yes," after 99 percent of polling stations were accounted for. Only about eight million of the 25 million Egyptians eligible to vote — a turnout of about 30 percent — cast their ballots, a significantly lower number than those who voted in most previous presidential and parliamentary elections.

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In the first round of voting, about 56 percent said "yes" to the charter. The turnout then was about 32 percent.

The local media has reported comparable results to the Brotherhood. The state-owned Al-Ahram newspaper said in its English language online version that 16.2 million cast their vote, and the constitution passed with a 63.96 percent.

The main opposition group, the National Salvation Front, said it would continue to challenge the referendum results, based on reports of violations and vote meddling it has registered.

"We are questioning the results," Khaled Dawoud, the front's spokesman, told The Associated Press. "We don't think the results reflect the true desires of the Egyptian people."

He said the opposition will be filing official complaints about observed irregularities such as supervisors who intentionally barred voters from casting their ballots, and a broader lack of blanket judicial supervision of the process. Many judges boycotted supervising the vote.

The new constitution will come into effect once official results are announced, expected Monday. When they are, Morsi is expected to call for the election of parliament's lower chamber, the more powerful of the legislature's two houses, no more than two months later.

The opposition said that even though it is challenging the results of the referendum, it will continue to prepare for the parliamentary elections.

Until the lower chamber is elected, the normally toothless upper Shura Council will have legislative powers.