JUSTIN BIEBER reduced a group of fans to tears after showing up unannounced at a preview of a new dance video game.

The girls were under the impression they were trying out Just Dance 4 as part of a focus group, and had no idea their 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 Bieber's prank, uploaded to YouTube.com, the youngsters begin screaming and crying when the Baby hitmaker walks into the room, with one hysteric teen even crying out, "Holy f**king s**t! Can I hug you?"

Bieber spent the rest of the afternoon posing for pictures and playing the game with his delighted devotees.