The city's biggest ever marijuana growing operation has been discovered in a Kwun Tong factory building, police said yesterday. The 2,500 sq ft greenhouse could produce 10kg of herbal cannabis a month, narcotics bureau police said. Officers found 1,113 plants and 25kg of finished product inside the Wai Yip Street unit during the raid that started at about 5pm on Tuesday. The area was fitted with 80 light fittings and a sprinkler system. Officers seized packaging equipment such as a vacuum-sealing machine and electronic scales. Bags of fertiliser were also found. The two local men arrested inside the unit, aged 29 and 42, were being held for questioning last night. No charges have been laid, police said. The raid was mounted after police received a tip-off and put the building under surveillance. 'It is the biggest marijuana-cultivating factory discovered in Hong Kong,' said an officer with the narcotics bureau. 'Investigations revealed that it [the facility] had been in operation since August last year and the finished products were for local consumption,' he said. The cannabis plants were at different growing phases ranging from 15cm to a metre tall. Officers are still investigating where the suspects obtained the plants as no cannabis seeds were found in the unit. Police said the suspects would likely be charged with cultivation, which carries a maximum penalty of 15 years' jail and a $100,000 fine.