Activists describe a tactic that has become known as the "circle of hell," Ahram Online reports. They say the men form two long lines and move through the crowds in the square.
Once the groups find one or two women alone they form a circle around them, trapping them.
Masa Amir, a researcher at Nazra for Feminist Studies, said the attackers continue to work together.
"One takes her shoes off, another pulls her trousers off, then someone else takes her phone and watch," she said.
Engy Ghozlan, the founder of Harassmap, said women sometimes find themselves surrounded with some men trying to help them and others to rape them: "The woman is confused and doesn't know who to trust."
Hatem Tallima, an activist and member of the Revolutionary Socialists, said assailants also pose as helpers. She said in large attacks men sometimes for three circles around a woman.
"The men in the circle immediately surrounding the woman begin to strip the girl," she said. "The second circle includes men who claim that they are helping the girl. The third circle try to distract the people in the square from what is happening."