Greater Manchester Police has arrested a custody officer on suspicion of sexual contact, misconduct and cyber flashing.
The independent police watchdog is investigating the claims, which it has described as serious.
A custodial detention officer (CDO) stationed at Bolton Police Station was arrested on Sunday following claims of sexual assault against a woman acting as a suitable adult for a man in custody, Greater Manchester Police said.
The CDO has been suspended and is on pre-charge conditional police bail
The force said the complaint was made in June, and related to alleged sexual misconduct which began in May.
CCTV footage from the police custody suite and other digital material has been seized, police said.
Detectives were obtaining full accounts from all relevant parties in the investigation, the force added.
An internal review is also being undertaken into how the complaint was initially handled, and the force had made a voluntary referral to the Independent Office for Police Conduct (IOPC), it said.
The IOPC said it was a mandatory referral.
IOPC regional director Catherine Bates said: “These are very serious allegations against someone in a position of authority and it is important for these to be fully investigated.
“In light of the nature of the complaint, this will be done by the IOPC, independently of the police.”