By Stephen Dowling
BBC News Online entertainment staff
For one night, some of Britain's youngest stars and some of their most respected elders were on equal footing - none of them had ever seen crowds like the ones waiting for the new Harry Potter film's premiere.
Director Alfonso Cuaron was there with daughter Bu
The stars of Harry Potter: The Prisoner of Azkaban were met with a mighty throng indeed - a crowd at least three times bigger than the one that met them in New York earlier this week.
Be they heavyweight Brit thespian statesmen or blossoming teenage star, the reaction from the stunned and fan-struck actors was unanimous - shellshock.
Even old hands like Sir Michael Gambon - Professor Dumbledore in the film - were taken aback by the fuss.
"It's overwhelming," he said. "I normally do quite cheap English films, and not one of them gets a premiere in Leicester Square."
Gambon described the film, directed by Mexican film-maker Alfonso Cuaron, as "easy" work - mostly because many of the cast, including Dame Maggie Smith and Robbie Coltrane, were "old friends".
Outside the Odeon, film crews and reporters from as far away as Australia screamed above the fans to get a soundbite. Flashguns bathed the entrance in dazzling light as ever-more-important cast members worked their way through autograph hunters to the venue.
Emma Watson - more than a little shocked by the response
UK premieres for some of the world's biggest blockbusters are routinely attracting more fans than their glitzy Stateside rivals - but even in a square that has seen the thousands wait and cheer for Peter Jackson and the Lord of the Rings in December, Sunday night's spectacle is awesome.
Pam Ferris, the British character actress who plays Aunt Marge in the film, was welcomed with the sight of 40 of her inflated likeness hovering over the crowd.
"Nobody told me they were going to be there!" she exclaimed to reporters after walking past the life-size balloons. "There are 40 'mes' in Leicester Square."
Parade of stars
Even Hollywood hardmen like Gary Oldman are defenceless in the face of Harry Potter mania.
Oldman, the star of films as diverse as Leon, The Fifth Element and True Romance, plays Sirius Black, the prisoner of the film's title.
"They just asked me to do it," he said matter-of-factly. "Anyone would want to play that part. I mean my name is Sirius Black.
"I've experienced the whole phenomenon through my kids. I have a five, 10 and 15-year-old. And now the kids have got me on their backs on T-shirts."
JK Rowling: "It's like The Beatles"
In all this glitz and glamour it is Oldman who takes time to thank the little people - the make-up, wardrobe and design people who have toiled on all three films.
The parade of stars is almost numbing.
Robbie Coltrane, sweating profusely, uses the arm of a TV reporter to mop his brow.
Alan Rickman is the epitome of leading man charm, flashing dazzling smiles and consistently ignoring his PR helper's increasingly desperate attempts to cut short his anecdotes.
Pop singer Natalie Imbruglia swept by with a young friend, while Claudia Schiffer ignored all and sundry with a haughty walk past the press throng. Dawson's Creek star Katie Holmes looked every inch a Hollywood A-list star.
Terry Gilliam, with a mischievous glint in his eye, said: "Harry Potter needs a good kick up the ass, and I think Alfonso's the guy to give it to him."
JK Rowling enters - in a very glam green gown - looking utterly blown away by the reception.
"It's very easy when you're at home to kid yourself it's your own private little thing.
"Out there it's like The Beatles," she said.
But as for any plot leaks for the new books - well, reporters are left clutching at straws. Rowling politely declines to give any details of what happens in the sixth and seventh as-yet-unpublished books.
But the biggest looks of surprise came from the trio of teenage stars.
This week has seen Daniel Radcliffe, Emma Watson and Rupert Grine touted as pin-ups - even amid claims that by the time the seventh and last film is being made they will be too old to play their characters.
Emma Watson played up to the cameras - giving an ear-piercing scream for Blue Peter cameras - and seemed every inch the young professional.
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"This is my favourite of the films, and my favourite of the books too," she enthused. But then there was a backwards glance to the roaring crowd. "This is a massive shock. I don't know how anyone gets used to this."
But Daniel Radcliffe - for whom the cheers had been longest and strongest - looked the most visibly unsettled.
As he talked to reporters, minutes after his crowdside wanderings, he twitched like a GI under mortar fire.
He showed steely determination in being a resolutely normal 14 year old, even though the growing acclaim will probably mean his remaining teenage years will be anything but.
He did have an answer to that question of whether he will stick around to the end.
"I think our producer was misquoted. He said there's a possibility that the three stars would not be there at the end. People have suddenly said 'they're definitely not going to be there at the end'. I think we'll see when we get there."