Harry Potter author JK Rowling spent the day "trying to avoid newspapers" as her first novel for grown-ups hit the bookshops, she told fans Thursday night.

But she is confident the book is "the best I can do," she added.

"The book is what I wanted it to be," she told about 900 fans at London's Southbank Centre on the day "The Casual Vacancy", a black comedy of village life, was published -- having already sold one million advance copies.

Kept closely under wraps until publication day, the book is a gritty tale involving sex and drug addiction that is widely expected to be Britain's best-selling fiction title this year. It is already topping the Amazon charts.

Set in the fictional village of Pagford in southwest England, it tells the story of the fight to fill a slot on the parish council after the incumbent's sudden death, and hinges on the fate of a squalid housing estate.

It took Rowling, 47, five years to complete and required another half-written children's novel to be put on the back burner, she told fans.

But writing it was "a lovely place to be -- there was so much pressure for the Potter books".

"I kept telling myself 'you don't have to publish this,'" Rowling said.

The book has met with a mixed reception from critics. Several of whom said it was dull in parts despite scenes of sex and drugs, and that Rowling's most vivid writing was on the familiar ground of children pitted against the power of adults.

Famously tough critic Michiko Kakutani wrote in the New York Times that "there is no magic in this book -- in terms of wizarding or in terms of narrative sorcery.

"Instead, this novel for adults is filled with a variety of people like Harry's aunt and uncle, Petunia and Vernon Dursley: self-absorbed, small-minded, snobbish and judgmental folks, whose stories neither engage nor transport us."

But in Britain, Allison Pearson wrote in the Daily Telegraph that "The Casual Vacancy" was "sometimes funny, often startlingly well observed, and full of cruelty and despair".

And Boyd Tonkin in the Independent called it a "song of freedom" as Rowling was able to abandon the constraints of children's writing following her seven books about the boy wizard.

Rowling revealed Thursday that her favourite characters in her own books were Fats -- a foul-mouthed schoolboy in the new novel -- and Dumbledore, the bearded wizard, wise but troubled, who is headmaster of Hogwarts in the Harry books.

She said that she would change "quite a few things" about the Harry Potter books if she could, and that the actors who played Harry and his friends Ron and Hermione in the blockbuster films were "all too good-looking".

In advice for budding writers, she said they should "get an agent!" and not always expect to feel inspired.

"Inspiration is clearly necessary, but then comes the long and hard work of writing," she said. "I definitely don't wake up and ask myself, 'Am I inspired today?'"

Danielle Salvatore, 19, a student from North Carolina, had paid £200 ($325, 250 euros) for a resold ticket to the event.

"It's worth it because JK Rowling inspired a lifetime of magic for me," she said.

Earlier in the day, as fans eagerly bought copies of the novel at Foyles bookshop in London's Charing Cross Road, general manager Sion Hamilton said the name alone would sell the book.

"Comparing it with other JK Rowling novels is difficult," she told AFP. "But even if we are far from Potter's universe, it's still from her pen."

Rowling told fans she had spent Thursday watching "Men in Black 3" with her children in an effort to avoid reviews, and that she "hated" the intense secrecy which has surrounded each of her books before publication day.

But the risk of online leaks had forced her publisher's hand, she said.

Rowling has previously said she left "the door ajar" for a return to the world of Harry Potter, although she said she was not intending to write any more books about the young wizard.

[Image via Agence France-Presse]