About 2,000 plants were found behind the former Blockbuster premises in Kirkgate after a phone call to police about suspicious activity in the area.
Inside the buildings were several crops of cannabis at various stages of growth, said police.
Insp Ian Williams said: "It is going to take us the best part of two days to remove what's in there."
Officers are appealing for information about vans being loaded or unloaded around the premises.

The number two seeds completed a 44-38 victory over China, who were champions in London four years ago.
Find out how to get into fencing with our special guide.
Romania, who finished sixth in the team epee in London, were in control throughout the game after winning the opening bout.
The Russian team secured their sixth bronze of these games with a narrow 37-31 victory over Estonia.
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Michelle Mellotte, 61, took the money from Michael McGrory in February 2010, saying it would be repaid that summer.
But, it was only returned following the start of a police investigation.
Ms Mellotte knew her patient was suffering from Alzheimer's when she asked him for the loan.
She went to his home to "ask him for a favour" and he agreed to sign a cheque for £10,000 with an endorsement that it would be paid back in the summer of 2010.
In April, Ms Mellotte admitted fraud by abuse of her role and was given a suspended prison sentence and was suspended from the medical register.
Last week, the ruling was endorsed following a hearing by the Medical Practitioners Tribunal Service in St James's Buildings in Manchester.
The tribunal noted that Ms Mellotte, who did not attend the hearing, did not oppose the GMC's case against he.
In written submissions, the former doctor's legal team stated she accepted the "appropriate sanction" was erasure.
Summarising the case against her, the tribunal chairman, Sean Ell, said: "The tribunal has determined that Dr Mellotte has put patients at unwarranted risk of harm, she has brought the profession into disrepute, she has breached fundamental tenets of the profession and her integrity cannot be relied upon."
She will no longer be able to practice as a doctor in the UK.

An ornate Roman coin, medieval silver pennies, a copper figurine, a thimble, window glass and a key were unearthed.
Foundations were also discovered, showing the original structure of Auckland Castle in County Durham was "significantly larger" than thought.
Curatorial director Dr Christopher Ferguson said he was "really excited to have uncovered such a major finding".
It suggests the castle was not created as a manor house for the Prince Bishops of Durham, as previously thought, but that it had always been a large castle complex.
A five-month excavation has been carried out ahead of a new museum extension in the castle's Scotland Wing, formerly used to hold Scottish prisoners of war.
Durham University archaeological services manager Peter Carne said the work had "enabled an unprecedented amount" of research into the castle and its grounds.
The new museum, due to open in 2019, will look at the history of faith in Britain.
A "major cannabis farm" has been found in disused buildings in Wakefield city centre, said West Yorkshire Police.
[NEXT_CONCEPT]
Romania's women won gold in the women's team epee to secure their country's first medal at Rio 2016.
[NEXT_CONCEPT]
A Fermanagh GP who dishonestly exploited and took advantage of a vulnerable elderly patient, when she failed to repay a £10,000 loan, has been struck off the medical register.
[NEXT_CONCEPT]
Archaeologists working at a 900-year-old castle have found "rare and unexpected" artefacts.