A total of 33 people have been arrested in Egypt in the past week because of their perceived sexual orientation, according to Amnesty International.

The arrests mark the "worst crackdown against LGBT people" in almost two decades, human rights activists say.

The crackdown started two weeks ago when several people were detained after holding up rainbow flags at a concert of Lebanese indie rock band Mashrou' Leila in Cairo on 22 September.

Egypt's public prosecutor announced an investigation into the "rainbow flag incident" on 25 September, detaining and questioning individuals who were caught on camera "raising the flag of homosexuals".

Homosexuality is not explicitly banned under Egyptian law, but LGBT people are routinely arrested on charges of "debauchery", "blasphemy" and "immorality" in the conservative country.

A 19-year-old man was arrested the day after the concert and sentenced last week to six years imprisonment for "debauchery". After completing his prison sentence, the man faces six years of probation, when he will be forced to spend up to 12 hours a day in a police station.

Since the first arrests, the national media has launched a "smear campaign" against LGBT people and police have detained 32 men and one woman because of their perceived sexual orientation, Amnesty International's Egypt researcher Mohamed Ahmed told IBTimes UK.

"They have tracked people down through their online dating applications and charged them with habitual debauchery and promoting sexual deviance," he said.

So far 18 people have been charged and will appear in court on 10 and 29 October. The other 15 remain in police custody. None of those arrested have been released.

"Every day we are hearing of more arrests," Ahmed said, adding that Egypt's Supreme Media Council announced this week that people who showed solidarity towards LGBT people could also face arrest.

Ahmed described the arrests as "the worst crackdown against LGBT people since 2001" when police raided a party on the luxury Queen boat and detained 52 gay Egyptian men.

The men were charged with "obscene behaviour" and "habitual debauchery" under the prostitution law and were subjected to beatings and medical examinations to determine whether they had had gay sex.

The Forensic Medical Authority has carried out anal examinations on five of those currently held in custody, according to Amnesty.

"Forced anal examinations are tantamount to torture – there is no scientific basis for such tests and they cannot be justified under any circumstances," Amnesty International's North Africa Campaigns Director Najia Bounaim said.

"The scale of the latest arrests highlights how dangerously entrenched homophobia is within the country. Instead of stepping up arrests and carrying out anal examinations, the authorities must urgently halt this ruthless crackdown and release all those arrested immediately and unconditionally," Bounaim said.

Mashrou' Leila said in a statement that they were "saddened" to see "another era of backwards tyranny creep over one of our most beloved countries and audiences".

"This crackdown is by no means separable from the suffocating atmosphere of fear and abuse experienced by all Egyptians on a daily basis, regardless of their sexual orientations.

"We denounce the demonisation and prosecution of victimless acts between consenting adults.

"It is sickening to think that all this hysteria has been generated over a couple of kids raising a piece of cloth that stands for love," the statement, which was shared on the band's Facebook page, read.