Potter author JK Rowling signed autographs for fans, some of whom had camped overnight to get a good spot.

The film, which is released on 15 July, will be followed by two more.

The final book in the series, Harry Potter and the Deathly Hallows, has been split into two parts for the cinema.

Radcliffe, 19, said he would miss playing Potter.

"It's going to be very, very sad to finish these movies but, at the same time, it will be exciting.

"There's more stuff to move on to so I'm not purely² depressed³ about finishing them."

Rupert Grint, 20, who plays Ron Weasley, attended the premiere after recovering from a mild case of swine flu.

"Swine flu is nothing really, I found out.

"It's just like normal flu, it's just that you are in bed for a while.

"Initially⁴, the words 'swine flu' was kind of scary, but it was easy."

Emma Watson, also 19, who was wearing a vintage（旧式的） Ossie Clark dress, said there had never been any romance between any of the films' three main stars.

"We have grown up together from the age of 10, to 19 or 20, and we are just like siblings⁵ - there is nothing. There is nothing between the three of us.

"Even among the other cast members, there hasn't been anything like that."

All the actors wore white ribbons on their wrists in memory of cast member Rob Knox - Marcus Belby in the film - who died after being stabbed outside a bar in Sidcup, Kent, in May 2008.

Jessie Cave, who plays Ron Weasley's love interest Lavender Brown, said: "Rob would have loved to be here tonight and it's just a shame that he's not."

Harry Potter and the Half-Blood Prince explores the past of the series' chief antagonist⁶（敌手，对手）, Lord Voldemort, as well as the growing pains of adolescence⁷（青春期）.

Stars who attended the premiere included TV chefs Jamie Oliver and Gordon Ramsay, TV host Graham Norton and actor Clive Owen.

Meanwhile, on Tuesday - ahead of the premiere - it emerged that Jamie Waylett, 19, who plays bully⁸ Vincent Crabbe in the Potter films, had been charged with growing cannabis（印度大麻） plants.

He will appear at City of Westminster Magistrates⁹' Court on 16 July.