The boys in One Direction, Harry Styles, Zayn Malik, Niall Horan, Louis Tomlinson and Liam Payne, were on hand at London's Leicester Square Tuesday evening for the world premiere of their new 3D concert documentary, One Direction: This Is Us.

As one would imagine, so were countless screaming fans, many of whom camped out to catch a glimpse of the lads. On the red carpet, Liam explained, "...It's absolutely crazy....We came here three years ago to go to a Harry Potter premiere and now we're at our own and this many people have turned up, we can't believe it."

As for the sheer number of fans who turned out, Liam said, "...Anything that has ever happened with us, they've always surprised us going above and beyond and they've just done it again today and it's amazing. They are so passionate and so dedicated and we're very lucky to have them, for sure."

Morgan Spurlock, known for documentaries like Super Size Me, directed the film, and he was excited to see the reaction. "More people are going to see this film on opening day worldwide than have seen all my films in theaters combined," he laughed. "As a filmmaker you want to make things that appeal to an audience. We make pop docs already and what's better pop than this?"

Simon Cowell, who discovered 1D on the U.K. version of The X Factor, was among the other celebrities who walked the red carpet. He had nothing but praise for the band, but when asked what the secret to their success was, he replied, "If I had the secret I'd bottle it."

Niall summed it all up, calling the premiere the "biggest night of our career...it's the biggest night of our lives."

One Direction: This Is Us opens in the U.S. on August 30.