Justin Bieber -- Alleged IMPOSTOR Arrested in Child Porno Sex Sting

A grown-ass man who allegedly POSED online as Justin Bieber has been arrested in Canada after cops say he threatened a 12-year-old girl into performing sex acts over the Internet.

Canadian authorities say 34-year-old Lee Moir has been charged with luring, manufacturing child pornography and extortion after allegedly posing as Bieber on Facebook ... and duping the girl into a video chat.

Cops say Moir threatened to harm the girl's family if she didn't engage in cyber-sexual activity. The girl reportedly complied.

The incident was reportedly investigated by U.S. law enforcement, which referred the matter over to Canada when they realized Moir was living in Toronto.

Moir was eventually busted by Canadian undercover cops who posed as a 14-year-old girl online. Officials say Moir engaged the officer in sexual conversations and requested a meeting.