Rihanna tried to break free to continue performing her hit "We Found Love," to no avail. Rather than getting security to take care of it, the singer clocked the fan with her microphone.
A YouTube clip shows the fan getting a firm grip on Rihanna's arm, resulting in the attempt to free herself.
It is unclear whether the fan was injured or where Rihanna scored her mic-hit, but several reports indicate the blow was to the fan's head.
A fan in the audience Tweeted, "[Rihanna] just hit someone with her mic on accident haha oops."
Rihanna responded that it was in self-defense: "Purpose! That [expletive] wouldn't let me go!" The tweet has since been deleted.
The fan could take legal action if he or she was badly injured.
Rihanna was already two hours late to the Diamonds Tour show after a night of partying with model Cara Delevigne.