Officers from the Greater Manchester police have sealed off an area in the centre of the city around a gay sauna.
It is thought their investigations focus on the Basement Sauna on Tariff St.
A rape reportedly took place at around 02:00 this morning.
Anyone with information has been asked to contact police, who have interviewed the victim of the alleged attack.
The sauna describes itself as “one of the largest gay and bisexual saunas in the north west” and is gay-owned and operated.
There is another Basement Sauna branch in Leeds.