Opposition spokesman Amr Hamzawy: "We are asking the commission to investigate the irregularities"
Egypt is due to announce the official results of a referendum on a controversial draft constitution.
Early unofficial results suggested more than 60% of voters said "yes" to the document, which is endorsed by Islamist President Mohammed Morsi.
But the opposition, which says the draft favours Islamists, is demanding an inquiry into alleged voting fraud.
Egypt has seen large and occasionally violent demonstrations by the opposing sides in the past few weeks.
If the constitution passes, elections must take place within three months and the deep polarisation in the country is likely to continue, the BBC's Bethany Bell in Cairo reports.
In the meantime, legislative powers would remain with President Morsi.
'Vote for stability'State media reports of the results following Saturday's second and final round of the referendum suggest that some 63% of voters had backed the charter. Turnout was low, estimated at 30%.
But the opposition National Salvation Front said on Sunday the vote had been marred by "fraud and violations".
These included polling stations opening late and Islamists seeking to influence voters, they said.
Spokesman Amr Hamzawy told a news conference the front as "asking the commission to investigate the irregularities before announcing official results".
However, another spokesman, Khaled Daoud, told the BBC the front would not contest the result.
The group has complained that there was not enough legal supervision of the referendum, after many judges boycotted the process.
The opposition said before the referendum that its campaign would continue in the wake of a "Yes" vote.
But the Freedom and Justice Party, the political wing of Mr Morsi's Muslim Brotherhood, said it hoped the "yes" vote would begin to heal divisions and bring stability.
It remains to be seen whether the announcement of the final results of the referendum will be delayed to allow an investigation of the opposition's claims, our correspondent says.
Critics of the draft constitution fails say it fails to protect the freedoms and human rights that they sought in the uprising that ended Hosni Mubarak's rule last year.
They accuse the president of pushing through a text that favours Islamists and does not sufficiently protect the rights of women or Christians, who make up about 10% of the population.
President Morsi's mainly Islamist supporters say the constitution will secure democracy and encourage stability.
The latest unrest began after Mr Morsi issued a decree on 22 November stripping the judiciary of the power to challenge his decisions.
After an outcry, the president revoked much of the decree, but he refused to back down on the draft constitution.
The text was rushed through by a constituent assembly dominated by Islamists.
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