Pop star Justin Bieber reduced a group of fans to tears after turning up unexpectedly to a preview of a new video game.

The girls were under the impression that they were trying out Just Dance 4 as part of a focus group at the company's offices, and had no idea their music idol was actually behind a one-way mirror checking out their moves, as they danced along to his music on the interactive Wii game.

In footage of Bieb's prank which was uploaded onto YouTube, the young teens, who are dancing to the 18-year-old's track 'Beauty And A Beat,' begin screaming and crying when the 'Boyfriend' hitmaker, dressed in a green hoody, makes his entrance into the room, with one hysteric fan even crying out:

''Holy f**king sh*t! Can I hug you?''

Canadian singer then spent the remainder of the afternoon posing for photos and joining in on the game with his thrilled fans.

Check out the reaction of Justin's fans in the clip below....

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