By G9ija

Hong Kong police arrested four men after a raid that uncovered explosives and a pipe bomb, government-backed broadcaster reported on Wednesday.

Police found what they called “protest-related materials” in a raid on Tuesday evening at a flat in Mong Kok, a densely populated residential area known for its outdoor markets and shopping areas.

Police also searched a house in Sheung Shui, a town close to Hong Kong’s border with mainland China, which they suspect was being used a laboratory for producing explosives, according to reporters.

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The pipe bomb said to contain around 40 grams of explosives was detonated by bomb disposal experts, which caused some damage to the building.

Police senior superintendent Chan Tin-chu said the bomb was “very powerful” and that a small explosion occurred while it was being defused.

“The shrapnel punched a hole in one of the lifts in the building,” Chan Tin-chu told reporters.

Police say that the bomb could have caused significant casualties.

Hong Kong has been plagued by protests for months, after a backlash against a now-defunct extradition bill, which critics say would allow the Communist Party in mainland China to single out dissidents.

They have included violent clashes between protesters and police.

Hong Kong was promised special rights and privileges under the “one country, two systems” framework that was put in place when the British handed the region back to China in 1997 after 150 years of colonial rule.