Rioters throw petrol bombs at the police in Causeway Bay, China's Hong Kong, on Aug. 31, 2019. Photo:Xinhua

Hong Kong police raid powerful pipe bomb lab in a residential building on Tuesday.

Police destroyed the laboratory where rioters made explosives. 

When a homemade bomb was detonated at the scene, the shrapnel punched a hole in one of the lifts.

Police arrested four core members believed to be from underground violent groups. Three of them are college students.