Journalists and human rights groups have accused Egyptian President Hosni Mubarak's ruling party of using security forces to assault pro-reform demonstrators and journalists during the referendum earlier this week.
The Egyptian Organization for Human Rights says it has asked the public prosecutor to investigate the attacks, several of which were against women.
A Human Rights Watch spokesman, Joe Stork, says the assaults show, in his words, "just how hollow the Mubarak government's rhetoric of reform really is." The United States has also condemned the violence.
An Egyptian government spokesman told Reuters news agency that the incidents had been overplayed by the media, and that they were very limited.
Egypt's opposition says the amendment to allow more than one presidential candidate, which easily passed Wednesday, has such restrictive rules that it will prevent any credible challenge to Mr. Mubarak if he decides to seek a fifth term.
Some information for this report provided by AP and Reuters.