Harry Styles let it all hang out in concert ... on accident!

On Saturday, the One Direction crooner fell victim to a practical joke during the group's live performance in London.

While Styles was singing the chorus of "You Don't Know You're Beautiful," band member Liam Payne decided to give their fans a very special treat ... when he pantsed Harry on stage!

Poor Harry, this isn't the first time he's been attacked during a concert -- in February he got hit with a shoe ... in the groin, while on stage.

The 19-year-old did his best attempt to smoothly pull up his trousers (while protecting his modesty), but he was definitely caught off guard by Liam.

Props to Payne, we're not sure how he was able to even get Harry's skinny jeans down -- those things are pretty tight.

Check out the hilarious clip above to see Styles' priceless reaction!

Hot Videos