A 39-year-old man was arrested at Wong Tai Sin Mass Transit Railway (MTR) station in Hong Kong's Kowloon on Sunday (March 29) for allegedly using an etomidate-laced e-vaporiser and assaulting an off-duty police officer who confronted him.

The incident reportedly took place in a train carriage at about 9am, reported The Standard.

Hong Kong police said the man, surnamed Wong, was approached by an off-duty police officer who identified himself to Wong and asked why he was vaping inside a train carriage.

Wong attempted to flee when the train reached Wong Tai Sin station. He is also alleged to have physically assaulted the off-duty officer who tried to restrain him.

He was later subdued with the assistance of MTR staff. 

A video of Wong's arrest posted on social media shows two plainclothes police officers armed with riot shields and four uniformed officers restraining and placing a pair of handcuffs on him.

The 39-year-old appeared breathless and dazed as he sat up against a wall after he was arrested. He also appeared to have sustained lacerations on his right knee. 

Hong Kong police said an e-vaporiser and two pods suspected to contain etomidate were seized from the man. They are estimated to have an estimated street value of about HKD560 (S$92).