WASHINGTON, Oct 20 (Reuters) - J.K. Rowling could hardly have caused a bigger sensation if she had arrived on a broomstick.

Children from 7 to 70 swarmed the fusty National Press Club on Wednesday, where Washington power brokers made way for a two-hour discussion of wizardry and its most famous current proponent - Rowling's literary creation Harry Potter.

Diners, some little taller than the linen-covered tables, heard the story of Harry's magic wand purchase ("It's really the wand that chooses the wizard") and Rowling's advice to parents of aspiring writers: "Don't tell them it's not realistic."

Rowling, the rags-to-riches British writer whose series on the schoolboy wizard has enthralled children across the world, has had fans lining up for hours in the rain for her signature on a tumultuous tour of the eastern United States.

She hardly needs a publicity tour.

Her three books exploded on the literary scene in the last year and now fill the top three spots in the New York Times bestseller list. Harry Potter has been on the cover of Time Magazine, which compared the books with childhood classics like "The Lion, the Witch and the Wardrobe" and "The Hobbit."

SEVEN MILLION BOOKS SOLD
More than 7 million copies have been sold in the United States alone, where Rowling has been credited by some with drawing millions of children into the pleasures of book-reading. The books have been translated into more than 25 languages.

Rowling, who started writing the books in Edinburgh cafes while she lived on welfare as a single mother just three years ago, earned 14.5 million pounds ($22 million) last year. There is a film in the making.

Buttoned-down Washington loosened its collar for a packed lunch at the Press Club, where children who have read the first three books in the promised seven-part series countless times sat spellbound next to sometimes befuddled parents.

The 34-year-old writer speaks in the succinct, enthusiastic style of her main characters, Harry and his friends Hermione, Ron and Neville, who attend Hogwarts school for wizards.

As a passage to read to an overflowing the Press Club ballroom, she chose the scene in which Harry is taken to a mysterious shadowy shop to buy his first magic wand - "holly and phoenix feather, 11 inches, nice and supple."

Briskly, Rowling read on: "Harry took the wand. He felt a sudden warmth in his fingers. He raised the wand above his head, brought it swishing down through the dusty air and a stream of red and gold sparks shot from the end like a firework, throwing dancing spots of light onto the walls."

IN TOUCH WITH HER YOUNG FANS
"She's not at all what I expected," said Iona Millership, an entranced 11-year-old who has read all three books several times and is quick to correct anyone confusing the arcane rules of the imaginary Hogwarts team sport of Quidditch.

"Her books are extraordinarily detailed, but she answers so briefly while saying all she wants to say."

Light and bright and engagingly child-like in her demeanour, flashing the thumbs-up sign to her young fans, Rowling has none of the attitude of a celebrity but is clearly not overwhelmed by the tidal wave of fame.

"Smart sassy; tough literate; gentle-mannered don't-mess-with-me," is how the Washington Post struggled to describe her after an interview.

Why are her books so popular? "I really don't want to analyse that," Rowling said. "I want to carry on writing the way I have. I don't want to put ingredient X in it."

"I had a very vivid fantasy life as a child." With a self-deprecating smile, she adds: "It's a bit worrying that I didn't outgrow it."

Although pressed by excited readers, she declined to give away future plot twists - each of the seven books tells the story of a year of Harry's schooling. She earlier had let slip that a central character will die, triggering horrified speculation among her readers.

But, slipping momentarily into the Washington jargon that haunts the press club, she would only say: "He has quite a full agenda coming up."