Egypt's new president, ex-army chief Abdel Fattah al-Sisi, has pledged to clamp down on sexual predators after a 19-year-old was sexually assaulted in Cairo's Tahrir Square, along with other women, during celebration to mark his election.

In one of the first public statements as head of state, Sisi called sexual violence an "unacceptable form of conduct, alien to the best principles of Egyptian culture".

"Sisi instructed the minister of the interior ... to vigorously enforce the law and take all necessary measures to combat sexual harassment," read the president's office statement.

The assault on the 19-year-old was recorded on camera and posted on social media. The footage showed a crowd of men surrounding the young woman, who was stripped naked and beaten, while police struggle to escort her out of Tahrir. The Interior ministry said that police had arrested seven men aged 16 to 49 after "they (sexually) harassed a number of girls during the celebrations in the Tahrir area".

Dozens of women say they have been attacked in Tahrir Square in the previous two years but this is one of the worst cases.

The incident happened after outgoing president Adly Mansour approved a decree criminalising sexual harassment in the country, a move that marks the first of its kind in the country.

The decree includes punishment of up to five years in jail for anyone found guilty of sexual harassment.

A 2013 UN study found that 99.3% of Egyptian women have suffered some form of sexual harassment, up to and including being sexually assaulted. Incidents against women mushroomed after the breakdown in the police force following the 2011 uprising that ousted dictator Hosni Mubarak.

Farah Shash, of the Nadeem Centre for the Rehabilitation of Victims of Violence, said in a report on "The culture here when it comes to sexual harassment is to blame the victim."

She said authorities in Egypt have never seen women's rights as a priority and females are reluctant to report incidents since the police are so unsympathetic.

Journalist Hania Moheeb, who was gang raped in Tahrir Square last January said: "I have considerably hope, not only in the present regime but in Egyptian girls because they have changed a lot.

"They are a lot more outspoken now and we'll fight for our rights.

"The denial this society has been living in for more than two decades is anything that has to end."