Lebanon's popular alternative rock band Mashrou' Leila has denounced a "witch-hunt" by Egyptian authorities against homosexuals after a rainbow flag was raised at one of the group's concerts in Cairo.

Nearly two dozen people have been arrested by Egyptian authorities since the flag, a symbol of the lesbian, gay, bisexual and transgender community, was waved by several people at a Mashrou' Leila concert on September 22.

The band, whose lead singer Hamed Sinno is openly gay, said they were "heartbroken" by the crackdown.

They said they had initially remained silent on the arrests for fear of "further inflaming the situation".

"It has however become rather apparent in the last 48 hours that the state apparatus is hell-bent on executing the most atrocious of human rights violations," the band said in a statement posted on its social media accounts late Monday.

"The state is currently rounding up kids and violating their bodies," the statement added, in reference to reports that authorities planned to carry out anal examinations on suspected homosexuals swept up in the detentions.

Homosexuality is not expressly outlawed in Egypt, but gays have previously been charged with debauchery in the deeply conservative Muslim society.

On Sunday, 17 men accused of homosexuality went on trial at a closed-door hearing facing charges of "debauchery" and "incitement to debauchery."

And the waving of the rainbow flag at Mashrou' Leila's concert provoked an outpouring of criticism in parts of Egypt's media.

"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 band wrote.

They called for an international campaign to pressure Egypt "to immediately halt its ongoing witch-hunt and release all detainees."

Egypt's conservative musicians syndicate has said Mashrou' Leila would not be authorised to perform again in the country.

The group has previously been banned from performing in the Jordanian capital.