The opposition accused President Mohamed Morsi's Muslim Brotherhood of attempted "vote rigging" in Saturday's referendum on a new constitution for Egypt.
The National Salvation Front, in a statement, expressed "deep concern... over the number of irregularities and violations in the holding of the referendum," charging it "points to a clear desire for vote rigging by the Muslim Brotherhood."
The statement, however, did not call for a boycott which the Front had threatened if it felt the referendum was unfair.
Instead it urged voters to reject the new constitution, which was largely drafted by Morsi's Islamist allies.
The Front sent a separate statement with a long list of alleged irregularities recorded.
The violations cited included unsealed ballot papers, Muslim Brotherhood members coercing people into voting for the charter, an absence of judges to monitor and polls being closed early.
In Cairo's Nasr City, where Islamists held a pro-constitution rally on Friday, a judge forbade Christians from voting, the Front said.
Egypt, the Arab world's most populous nation, is split over the draft charter, which is being decided in a staggered referendum being held Saturday and a week later.
Morsi's camp argues the text needs to be adopted to stem the instability which has endured since the overthrow early last year of Hosni Mubarak.
But the secular-leaning opposition sees the proposed constitution as undercutting human rights, especially for women and religious minorities, and boosting military powers to usher in sharia-style laws wanted by the Islamists.
Egypt has weathered three weeks of protests against Morsi and the draft charter. Last week, clashes between pro- and anti-Morsi demonstrators in Cairo left eight people dead and hundreds injured.