Police have uncovered a large scale cannabis factory covering five floors of a vacant high street shop in Chepstow.
Residents in the area saw Gwent Police firearms officers smashing the windows of the former TUI site, which is the unit next to the WHSmith.
They also claim to have seen a man running away before getting arrested.
Many had been smelling cannabis in the area for a while, and so despite not knowing for certain what was happening at the time of the raid, most suspected a drug farm.
A man allegedly heard a policeman say it was "the largest they had ever seen."
Gwent Police were working alongside the scientific investigations unit and the National Grid at the scene.
A spokesperson for Gwent Police said: "Gwent Police carried out a warrant, under the Misuse of Drugs Act, at a disused commercial premises on High Street, Chepstow, at around 8.30am on Monday 4 November.
"We uncovered a large cannabis cultivation across five floors and arrested a man on the suspicion of the production of class B controlled drugs – cannabis.
"The man, aged 36, remains in police custody at this time, as our enquiries continue."
PC Hywel Evans, the officer in the case, said: "This cannabis cultivation in Chepstow was discovered following reports of suspicious activity inside the building.
"While producing cannabis may not seem like a serious offence, it can often be linked to sophisticated organised crime groups.
“Every cannabis factory we dismantle helps disrupt these operations, the plants are stopped from being sold into our communities and the profits are prevented from going on to fund other crime.
“The public play an important role in providing information and we encourage anyone with concerns about illegal drugs in their area to get in touch so that we can take action."
"If you have any information or CCTV footage that could assist our investigation, contact us via the website, call 101, or direct message us on our Facebook or X pages, quoting log reference 2400363128.
"Alternatively, you can contact Crimestoppers, anonymously, on 0800 555 111, with information."
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