Having uncovered the "largest and most sophisticated" illegal marijuana growing operation, Ontario police say the massive pot factory cleverly concealed in a former brewery is evidence the drug has reached "epidemic proportions."

The marijuana grow operation found at a former Molson brewery in Barrie, Ontario, in plain view of the busy Highway 400, held tens of thousands of marijuana plants with a street value of tens of millions of dollars.

"This is not a ma-and-pa operation," Barrie police Chief Wayne Frechette told a news conference Monday, describing the findings of a weekend raid.

"There were millions invested alone in just the equipment," said Detective Chief Superintendent Vaughn Collins of the Ontario Provincial Police. "This particular marijuana factory is the largest and most sophisticated I'm aware of in Canada.

"It took us two days to clear the building, it will take weeks to conclude our forensic work and weeks to conclude our investigation," he added.

More than 100 police officers moved into the operation -- with kilometres of irrigation piping and 1,000 hydroponic lights covering an area the size of a football field -- early Saturday. They raided a second location nearby that evening.

Both facilities were being run on a 24-hour-a-day basis, officials said, and would be capable of producing three or four crops a year -- a marijuana net worth hundreds of millions of dollars.

"Welcome to what, as of Friday, appears to have become the marijuana capital of Huronia," said Frechette.

Nine people have been charged, eight of whom face one count each of production of a controlled substance and possession for the purpose of trafficking.

The ninth was also charged with one count of production of a controlled substance as well as one charge each of possession of cocaine, possession of ecstasy and possession for the purpose of trafficking.

They are all scheduled to appear in a Barrie court on Tuesday.

Police say the marijuana plants were being run as round-the-clock factory operations. Both facilities featured living accommodations with beds for as many as 50 people, as well as televisions, fridges and stoves.

Vaughn Collins told reporters the discovery is just the tip of an iceberg.

"Commercial marijuana factories have reached epidemic proportions in Ontario, they are in every community and most are controlled by organized crime," Collins said.

Molson, Canada's largest brewing company, shut the Barrie plant down just over three years ago. Four hundred people lost their jobs.

Since then, it's been leased to industrial tenants. It isn't known yet how such a large operation managed to exist for so long.

There are an estimated 15,000 illegal grow operations in Canada. Collins said much of that marijuana is shipped to the U.S., where it is traded for cocaine.