Human Rights Watch has called on Egypt to investigate what it labels state-sponsored brutality against opposition demonstrators.
Pro-government and opposition supporters confronted each other
The group said plain clothes security officers and government supporters beat protesters during Wednesday's vote on partial electoral reform.
Egyptian opposition groups also contest government figures of a 51% turnout.
Official figures show that 83% of those who voted were in favour of limited multi-candidate presidential elections.
The opposition, which had called for a boycott, said the government had forced public sector workers to vote and even then turnout was not more than 20%.
The government hailed the result as a huge victory for democracy, saying Egyptians had given their overwhelming support so they could choose their future president from several candidates for the first time in their history.
There have been widespread reports of opposition protesters being beaten and intimidated by government supporters and security men on the day of the referendum.
In some cases, police reportedly stood by as demonstrators were beaten by supporters of President Hosni Mubarak's party.
Reports say women were singled out by loyalist attackers
There were also reports of women being targeted for beatings and their clothes being ripped off.
"We were shocked when our members were beaten and dragged on the streets. Some female colleagues were subjected to humiliation of a sexual nature. It was completely shocking," Kifaya opposition movement spokesman George Ishak said.
"The police and ruling-party assaults on pro-reform advocates yesterday shows just how hollow the Mubarak government's rhetoric of reform really is," Joe Stork, deputy Middle East director of Human Rights Watch, said.
"At a minimum, the president should appoint people with unquestioned integrity to investigate this state-sanctioned brutality."
The government has not responded yet to these allegations.
But the United States, which had welcomed the constitutional reform, has voiced concern.
President George W Bush said on Thursday that violence against peaceful demonstrators in Egypt did not conform to democratic norms.
The opposition had called for a boycott of the vote because, it said, the amendment was designed to prevent any real competition to Mr Mubarak, who has ruled Egypt for nearly a quarter of a century and may still stand for another six-year term in the presidential election due in September.