The Egyptian National Council for Human Rights said it forwarded more than 100 complaints on to the Supreme Electoral Committee regarding attempts by supporters of former President Mohamed Morsi to influence voters, Egyptian news website Ahram Online reports.
The Arabic Network for Human Rights Information, for its part, said there were several reported instances where voters were pressured to cast a "yes" vote for a draft constitution meant to replace the one adopted during Morsi's tenure.
Morsi was ousted by the Egyptian military in July. His administration was accused of favoring the Islamic ideologies of the Muslim Brotherhood, of which he's a member.
The Muslim Brotherhood called on its supporters to boycott the referendum. The movement described Morsi's ouster as a military coup.
Ahram Online reports there's a heavy police presence throughout the country as the vote entered its second day Wednesday. Violent clashes were reported in Egypt during the first day of voting Tuesday.
Results of the referendum are expected Thursday.