"This has grave implications for prospects of democracy in Egypt in the medium term and makes it hard for alternative political voices to contest power in the long term", she said in an emailed comment to Reuters.
Sherif said that the commission was monitoring the vote for any violations, which he said could constitute "electoral crimes".
Polling stations nationwide started receiving voters for the second day to cast their ballots in the referendum on the constitutional amendments on Sunday at 9 am.
Egypt's 596-member parliament, dominated by Sisi's supporters, approved the amendments on Tuesday, voting by 531 to 22 in favour.
However, organized opposition to the referendum was nearly nonexistent, with many leading public figures, businesses and media houses firmly in league with the current government.
By Sunday, local pro-government news hailed the turn-out in many polling stations as "huge", particularly drawing female voters. Reports focused on Mr Sisi's mega-projects, such as a new capital city outside Cairo, which he says will create jobs and economic growth.
The move comes eight years after a pro-democracy uprising ended Hosni Mubarak's three-decade rule, and almost six years after Sisi led a popular military overthrow of the country's first freely elected President Mohamed Morsi, after protests against his rule.
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Ms McKee had been on the scene in the Creggan area of the Northern Irish city as police and rioters clashed in violent attacks. Saoradh, translating to liberation in Irish, is a grouping which has the support of the dissident republican group the New IRA.
The proposed changes were blasted by critics as another major step toward authoritarian rule.
His government has been widely criticised by human rights groups for the repression of political opponents.
The Civil Democratic Movement, a coalition of liberal and left-leaning parties, has urged people to participate in the referendum by voting "No".
The sweeping changes would extend his current term until 2024 and would also give him the right to stand again for re-election for another six-year term.
The opposition also said they were banned from hanging banners in the streets to call on voters to reject the amendments and said they have only used social media to reach out to people.
El-Sissi was elected president in 2014, and re-elected a year ago after all potentially serious challengers were either jailed or pressured to exit the race.
In the last three years, over 15,000 civilians, including children, have been referred to military prosecution in Egypt, according to Human Rights Watch.
The referendum also proposes other changes to the five-year-old constitution, among them the creation of a second parliamentary chamber and a quota ensuring at least 25 percent of lawmakers are women. All three had been dropped from Egypt's constitution after the 2011 revolution.