CAIRO — Egypt’s opposition said Sunday it will keep fighting the Islamist-backed constitution after the Muslim Brotherhood, the main group backing the charter, said it passed with a 64 percent ‘‘yes’’ vote in a referendum.
The opposition alleged vote fraud and demanded an investigation — a sign that the referendum will not end the turmoil that has roiled this country for nearly two years since the uprising that ousted authoritarian leader Hosni Mubarak.
Many Egyptians, especially the tens of millions who live in extreme poverty, had hoped the new constitution might bring more stability to the country.
A heated political debate over the past month leading up to the referendum at times erupted into deadly street battles.
There were no mass opposition demonstrations on Sunday after the unofficial results came out.
Renewed violence and political tensions have further imperiled Egypt’s precarious economy, reeling from dwindling resources and a cash-strapped government whose plans to borrow from the International Monetary Fund had to be pushed back because of the turmoil.
The Finance Ministry said Sunday the budget deficit reached $13 billion in the five months from July to November, about 4.5 percent higher than the same period last year.
Official results of the referendum are not expected until Monday.
If the numbers are confirmed, it will be a victory for President Mohammed Morsi, who is from the Brotherhood.
But the opposition allegations are expected to prolong the fight.
Opposition leaders also are expected to challenge new laws issued on the basis of the constitution as well as Morsi’s economic policies.
‘‘The referendum is not the end game. It is only a battle in this long struggle for the future of Egypt,’’ said the National Salvation Front, the main opposition group. ‘‘We will not allow a change to the identity of Egypt or the return of the age of tyranny.’’
The opposition said the new constitution seeks to enshrine Islamic rule in Egypt and accuses the Islamists of trying to monopolize power.
Critics say it does not sufficiently protect the rights of women and minority groups and empowers Muslim clerics by giving them a say over legislation.
Some articles were also seen as tailored to get rid of Islamists’ enemies and undermine the freedom of labor unions.