LONDON, July 8 (Reuters) - The woman who gave birth to Harry Potter said she was amazed by the "complete madness" surrounding the latest instalment of the young wizard's adventures as it went on sale on Saturday.

"It's complete madness...Being published is what I always dreamed of; never in my wildest dreams did I imagine this," JK Rowling said, clutching a copy of "Harry Potter and The Goblet of Fire" at King's Cross railway station in London. Hundreds of Potter fans thronged platform nine-and-three-quarters to see Rowling chug away on the Hogwarts Express, the same way Harry travels to Hogwarts School of Witchcraft and Wizardry.

She called the Potter phenomenon "strange but wonderful", as her latest tale of the young wizard's battle with the forces of darkness literally vanished from Britain's bookstores the day it was published.

HUNDREDS OF FANS

Rowling, a 34-year-old single mother who lives in Edinburgh, rushed up and down the platform trying to keep hundreds of fans of all ages happy by signing copies of the new 640-page book.

And she seemed oblivious to a scuffle between a parent and one of the hordes of photographers scrabbling for a shot of her.

Rowling arrived on the platform in a pale blue Ford Anglia, similar to the flying car Potter used in the second book, The Chamber of Secrets.

But to her disappointment, the Hogwarts Express which was due to carry her north on a book-signing tour through Britain was closer to burgundy than the more magical purple she had envisaged.

"The platform's very close to the one in the book, but the train's the wrong colour. We won't quibble though," she said, as acolytes from her publisher Bloomsbury milled around her.

As she chugged out of London, bookshops were struggling to keep shelves stocked with the fourth volume in the hugely successful series.

Leading booksellers said the new tales of the young orphaned wizard in a fantastic, parallel world invisible to "Muggles" - ordinary common folk - were selling like hot cakes.

30 MILLION COPIES

The first three Potter books have sold more than 30 million copies around the world and have been translated into 31 languages. The bespectacled schoolboy with the lightning-bolt scar on his forehead has also graced the cover of Time magazine.

After Potter had spent almost 100 weeks on the New York Times bestseller list, Hollywood decided to put him on to the silver screen. Some 40,000 young hopefuls applied to be the celluloid Potter.

"Make sure Harry Potter's not American," one young fan shouted to Rowling at King's Cross.

"I'm fighting," she called back, adding that she was "very excited" about seeing Harry on film playing his favourite game, a cross between hockey and rugby on flying broomsticks.

"I can't wait to see quidditch, just like everyone else," she said.

The Potter feeding frenzy is already showing signs of moving on to the fifth instalment in what is planned as a seven-book series.

But when asked what she would do when the furore subsided, Rowling replied without hesitation: "I'll probably cry a bit and then have some peace." (Additional reporting by Kathrin Ebeling).