The singers got emotional during a rendition of their 2012 ballad 'Little Things', while playing a headline show at the SECC on Tuesday night (February 26).
> One Direction live at The O2, London - review
The moment happened on the same night Harry Styles was hit in the groin with a shoe thrown by a fan in the crowd.
The group had just performed 'Last First Kiss' at the arena, and were taking part in a question and answer session when an object landed on stage.
One Direction scored their third UK number one single at the weekend with their Comic Relief track 'One Way or Another (Teenage Kicks)'.
Watch fan footage of Niall Horan and Zayn Malik crying during 'Little Things' below: