Officers found 826 plants in the basement of a non-residential property on Throgmorton Street in the City of London -- the financial district of the capital and home of the Bank of England, as well as attractions including St Paul's Cathedral and Leadenhall Market.
The City of London, which is also known as the Square Mile, is just 1.12 square miles in size but houses many important historic and modern buildings, having been an ancient center of London stretching back to the Roman era.
The police force said it was the first cannabis factory found in the City of London, and suggested that criminals had taken advantage of reduced footfall owing to the UK's ongoing coronavirus lockdown restrictions.
Officers discovered the illegal operation on January 13, and entered the premises with a warrant on January 14.
"This is the first cannabis factory in the City, no doubt being set up in response to fewer people being out and about during the pandemic who might have noticed any unusual activity," said temporary detective inspector Andy Spooner, who is leading the investigation.
"However, this demonstrates that City of London Police continues to actively police the Square Mile, bearing down on any crime committed here."
Police said they have arrested two people.