Justin Bieber surprised some strangers on Monday (January 19) by paying for their restaurant bill.

The singer had tweeted earlier in the day that people should go and do something for their "fellow man" in honour of Martin Luther King Day.

Yahoo editor Kwala Mandel later took to the social networking website to say that she had just witnessed the Canadian paying for a random bill in a restaurant.

She wrote: "Saw @justinbieber at a pizzeria. When he left he picked up the tab for strangers sitting next to him. Good Guy Justin Bieber."

Mandel added to Bieber directly: "You created a chain reaction! Those guys turned around, picked up the tab for two others. Paid it forward :)."

The singer replied to say: "Thanks :) #giveback."

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Watch Justin Bieber in action below: