(CNN) -- Sexual violence against female protestors at Cairo's Tahrir Square appears to be increasing, an activist group devoted to end such assaults said Tuesday.
Operation Anti-Sexual Harassment/Assault said it intervened in 15 of 19 reported sexual assaults on Friday when 10,000 people gathered in the square on the two-year anniversary of the start of the revolution that ousted President Hosni Mubarak.
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The organization said some of the cases involved life-threatening violence where the attackers used knives or other weapons. It said some of its members also were attacked during rescue attempts.
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"This phenomenon requires urgent attention and treatment, and is linked to the broader social problem of endemic and daily sexual harassment and assault of women," Operation Anti-Sexual Harassment/Assault said.
U.N. High Commissioner for Human Rights Navi Pillay called for Egypt President Mohamed Morsy to do something about the assaults and other violence against demonstrators.
"It is unacceptable and a dereliction of duty not to intervene when protesters are being attacked by thugs and when women are being raped and sexually assaulted," she said in a written statement.
The streets around Cairo's Tahrir Square have been the sites of protests for days, including Friday's violent clashes.
At least 29 people were treated for cuts, broken bones and birdshot injuries, Health Ministry spokesman Khaled El Khatib said. Six police officers were also hurt in the disorder near Tahrir Square, the Interior Ministry said.