One Direction film premiere kick-off in London

Thousands of screaming teenage girls swamped London's Leicester Square on Tuesday as British boyband One Direction launched their first film with a glitzy red carpet premiere.

Fans have camped out since Sunday to secure prime spots, hoping to catch a glimpse of the five singers as they arrived for the first screening of "One Direction: This Is Us".

The film, made by "Super Size Me" director Morgan Spurlock, shows the group in concert at London's O2 Arena, along with fly-on-the-wall footage of life inside one of the world's most famous bands.

They have topped the music charts on both sides of the Atlantic, becoming the first British act to top the Billboard chart with their debut album.

The boys in the band are Irish-born Niall Horan, 19, Zayn Malik, 20, Liam Payne, 19, Harry Styles, 19, and Louis Tomlinson, 21.

"This is unreal," said Styles at the premiere.

Horan added: "This is amazing. We're really excited to be here. We can't believe we are here.

"This is just unbelievable. The last few years have been amazing. This is what we've been dreaming of."

With thousands of fans already in place, organisers were worried that tens of thousands more may try to descend on Leicester Square, the cinema-packed plaza of UK film premieres.

They set up special areas so fans, nicknamed "Directioners", could see the band arriving but urged others to follow the action online.

"The red carpet is being streamed live all over the world, so they can get the best views from the comfort of their own homes," organisers said.

Fans sang and chanted to pass the time, holding up dolls of the band members, flags, home-made placards and messages including: "Harry you wanna marry me?"

Emily Gee, 16, from Solihull in central England, camped out while her mother stayed in a nearby hotel.

"I didn't sleep at all. I'm completely shattered," she said.

"I don't know how to explain in words how I feel. I'm buzzing -- I can't wait.

"I like them all equally but I love Niall."

The band were put together in 2010 during the television singing competition "The X Factor". They came third in the public vote but the show's mogul Simon Cowell signed them up and they have gone on to become global superstars.

Their hits include "What Makes You Beautiful" from their debut album "Up All Night", and "Live While We're Young" from their follow-up album "Take Me Home".

Iraia Landaluze, 18, is visiting London with her family from Spain and they joined the throng in Leicester Square.

"I have come down as I'm a big fan. I like One Direction as they are normal boys. Liam's my favourite," she said.

Of the crowds, she added: "It's crazy."

One scene from the film features Payne's father talking about how much he misses his son because of the constant touring.

The singer told BBC television: "For me that was a bit difficult to watch. He's always so happy when I see him and happy to have me home; to actually see what he's like when I'm not there was a bit mad."

Tomlinson agreed it was "tough" for their parents.

"It kind of just happened out of the blue and our mums just had to wave goodbye to us on 'The X Factor' so I think it was a lot for them to take in at first," he said.

Spurlock said fans would enjoy seeing how "normal" the band are.

"You see five guys who are the same five guys they were three years ago," he said.