CAIRO -- Egypt's opposition on Sunday charged that fraud was committed during a referendum on the nation's Islamist-backed constitution, which preliminary results indicated had been approved by voters. The dispute augured more ill will and public confrontation between Muslim Brotherhood supporters and their opponents.

The constitution, which went before voters Saturday in a second and final round of balloting, was approved with 64 percent of the vote, according to an initial count, the state news agency reported.

Leaders of the National Salvation Front accused Islamists of systematic fraud and urged the government to not put the constitution into effect until after an investigation.

"We documented a set of irregularities that are clearly evidence of systemic violations," said Amr Hamzawy, an NSF leader and political science professor. "We would like the responsible party to investigate and report the conclusions before the final results of the referendum are announced," he added, referring to Egypt's High Election Commission.

The NSF said it had compiled a list of 1,500 violations in the referendum's first round on Dec. 15 and said a similar number of abuses occurred Saturday. Charges include voters being turned away and judges who served as observers turning up late at voting centers where the constitution was expected to fail. NSF leaders said that the constitution amounted to President Mohammed Morsi's first step in a plan to build an Islamist dictatorship.

Hamdeen Sabahi, a NSF leader and former presidential candidate, said the coalition would continue to back demonstrations to pressure the government.

Sabahi said the non-Islamist parties looked to elections as the place to defeat Morsi's ambitions, but added that they have not decidided whether to compete in parliamentary elections early next year.