At least 19 women were sexually assaulted or raped in Cairo's Tahrir Square during the second anniversary of Egypt's uprising.
A 19-year-old woman was attacked with a knife while others were assaulted on Friday, Dalia Abdel-Hamid, said a researcher at the Egyptian Initiative for Personal Rights.
Ms Abdel-Hamid and other women's rights activists have repeatedly highlighted sexual harassment and assaults in Egypt, saying the problem has grown increasingly widespread and violent since the uprising that ousted President Hosni Mubarak on February 11, 2011.
"The assaults were the largest and most intense as far as we know since the revolution," Ms Abdel-Hamid said. "The attackers were using weapons and sometimes there were four concurrent mob harassments in the street that could be seen from the top of a balcony."
Male volunteers who worked to rescue women in the square, also came under attack.