Detectives investigating slavery and kidnap allegations have arrested four men.
An operation involving three police forces saw warrants executed at five addresses in Bristol and Cardiff this morning.
Four men from the Rumney area of the Welsh capital were arrested, Gwent police said.
Police working on Operation Imperial are probing alleged offences of slavery, forced labour, false imprisonment, kidnap and assault.
Lead officer Detective Superintendent Paul Griffiths said: "The alleged offences we are investigating are extremely serious and we would urge anyone with any information which could assist to contact the Imperial team directly by dialling 01633 647174.
"Alternatively if someone has information and would rather not pass this directly to the police then I would urge them to contact Crimestoppers, which is independent of the police and guarantees complete anonymity on 0800 555 111."
Three men - aged 57, 37 and 33 - were held on suspicion of knowingly or conspiring to hold a person in slavery, servitude or requiring another person to perform forced or compulsory labour, kidnap, false imprisonment and assault.
A fourth man, aged 34, was arrested on suspicion of knowingly or conspiring to hold a person in slavery, servitude or requiring another person to perform forced or compulsory labour and assault.