Justin Bieber filmed punching fan in the mouth

Pop star Justin Bieber has struck a young fan from a moving car window, leaving him with a bleeding mouth.

Footage of the incident, which happened in Barcelona, Spain on Tuesday night (local time), shows the 'Sorry' singer being driven in a car as the fan reaches through an open window to greet his idol.

Bieber's fist suddenly comes flying out of the window and appears to hit the fan in the face, before he reels away and shows the camera what looks like a cut lip.

It's not the first time the often-troubled former child singer has lashed out violently.

In 2013, he was restrained by his own security while screaming "I'll f**king beat the f**k out of you" at paparazzi in the UK.

Newshub.