CAIRO — Amid a growing number of brutal attacks on women protesters in Egypt, one stood out: A mob of men on Cairo’s Tahrir Square raped a 19-year-old woman with a sharp object, cutting her genitals in an attack that forced her to undergo emergency surgery.

The assault was one of the worst in a string of attacks over the past year in which women have been stripped, groped and raped at demonstrations in Egypt. Last week alone, more than two dozen new sexual attacks were reported — a wave that activists call the worst in years and describe as the darkest stain on the country’s opposition street movement.

In response, several groups have arisen to protect female demonstrators. On Friday, men and women clad in bright neon vests and hardhats patrolling Cairo’s iconic square were part of the latest organization to emerge: Tahrir Bodyguard.

Soraya Bahgat said she founded the group using social media after seeing television footage in November of a mob of men attacking a woman and tearing off her clothes. She had been on the way to a demonstration at Tahrir herself, but instead stayed in, gripped with fear.

“It was sickening. They were dragging her through the street,” said the 29-year-old human resources manager. “I couldn’t imagine something so horrific, and something that fundamentally would keep women from exercising their right to assembly like anyone else.”

Such is the concern that the United Nations on Thursday demanded authorities to act to bring perpetrators to justice, saying it had reports of 25 sexual assaults on women in Tahrir rallies last week. Operation Anti-Sexual Harassment/Assault, which also patrols the square, reported 19 incidents on Jan. 25, the second anniversary of the start of the uprising that toppled Hosni Mubarak.