A total of 33 people have been arrested in Egypt as part of a massive new crackdown on perceived homosexuality, following the raising of a gay pride flag at a music concert in Cairo.

The unfurling of an LGBT rainbow flag at the 23 September gig of Lebanese alternative rock band Mashrou’ Leila was met with fury by local media after it was captured in photos, leading the public prosecutor to announce an investigation into the incident.

According to a statement from Amnesty International on Monday, 22 arrests have been made in the last three days, sparking fears the wave of detentions is not over yet.

Two men were arrested in relation to the public appearance of the flag, one of whom was later released.

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The other 31 arrests in Cairo, Giza and Damietta have all been made in the last nine days, for “promoting sexual deviancy” and “debauchery”: the charges usually brought against perceived gay individuals.

Of those detained, 32 are men. The person believed to have raised the flag in a rare show of gay rights solidarity in the conservative country is a woman – the first to be arrested on such charges in years.

16 men were tried on Sunday and await a verdict on 29 October. One man has already been sentenced to six years in jail on similar charges.

The clampdown is the most severe state attack on LGBT rights since 2001, when police arrested 52 men in a raid on a floating disco called the Queen Boat.

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“The scale of the latest arrests highlights how dangerously entrenched homophobia is within the country,” Najia Bounaim, Amnesty International’s North Africa Campaigns director, said in a statement.

“The Egyptian authorities’ announcement that they are investigating the rainbow flag incident as a criminal act is utterly absurd. No one should be punished for expressing solidarity with LGBT individuals or based on their perceived sexual orientation,” she continued.

“The authorities must urgently halt this ruthless crackdown and release all those arrested immediately and unconditionally.”

Beirut-based Mashrou’ Leila is a wildly popular band, in part because of their stance on social issues such as LGBT and women’s rights. Lead singer Hamed Sinno is one of the only openly gay artists in the region, which has often caused problems for the band during tours to conservative countries.

The band has reportedly now been banned from Egypt, according to the country’s Syndicate of Musical Professions, over its “abnormal” art.

While there are no specific laws against homosexuality in Egypt, discrimination against individuals perceived to be LGBT is rife, as are arrests on debauchery and immorality charges.

Detained people are often subject to anal examinations to determine whether they have had gay sex, which rights groups such as Human Rights Watch say are against human rights law and amount to torture.