The latest Harry Potter book is set to break sales records, as shops reported "barmy bookselling" in its first two days.
JK Rowling gave fans their copies at a bookshop in Edinburgh
JK Rowling's Harry Potter and the Order of the Phoenix is the fastest-selling book ever, according to several chains.
Hundreds of bookshops across the UK opened at midnight on Friday night, while stores from New York to Beijing have also been besieged.
Tesco sold an average number of 220 copies a minute in the first 24 hours, while WHSmith estimated a rate of eight sold per second.
Fans have had to wait three years for this fifth instalment about the exploits of the boy wizard Harry and his boarding school friends Ron and Hermione.
A Tesco spokeswoman said that it sold 317,400 copies in the first 24 hours - compared to 42,000 copies of the fourth Harry Potter
book, The Goblet of Fire, in its first week.
A spokeswoman for the WHSmith chain said: "We anticipated it was going to be massive, but it's the fastest selling book we've ever had."
The chain said it expected the new novel to be 10 times more popular than the previous instalment.
Philip Bell, manager of Blackwells flagship store in Oxford, said he was on course to sell about 1,000 copies on Saturday - the same amount that the hardback version of the fourth Potter book sold in its first year.
"That's really caught us by surprise," he said. "We thought 1,000 copies would see us through the first week at least.
"I've been in the business 18 years and I've never seen anything like this. It's absolutely barmy."
A spokeswoman for the Waterstone's stores said sales levels were "as much if not more" than expected.
And internet retailer Amazon revealed that it has taken orders for 1.3 million books around the world.
Some 420,000 of those orders were placed with the site's UK arm, and Royal Mail laid on extra vans to deliver almost 500,000 copies on Saturday.
Harry Potter and the Order of the Phoenix reveals which character dies - a much-guarded secret - and will keep readers occupied with its 766 pages.
BBC News Online's Darren Waters wrote in one of the world's first reviews: "Rowling has succeeded in crafting a book that will disappoint no true Potter fan."
Rowling herself attended a midnight launch of her book in Edinburgh.
HARRY POTTER AND THE ORDER OF THE PHOENIX
13 million copies printed
1.3 million copies sent out by Amazon worldwide
Royal Mail delivered 500,000 on Saturday
Expected to make JK Rowling £30m
Previous Potter novels have sold 200 million
"When Goblet of Fire was published I was desperate to go into a bookshop at midnight and see children's reactions, so this time I'm really pleased I could," she said.
At one midnight party at a shop in King's Cross, London, the first fan in the queue was Gillian Ammerton, who dressed as a witch with a broomstick and a toy owl.
"I am the biggest Harry Potter fan, I love him," she said.
"I have two copies, one is signed which I'm going to save and I have another one to read. I think it will take me about three days."
Rowling is expected to make £30m from sales of the book, with 13 million copies printed so far.
Harry Potter and the Goblet of Fire, the fourth novel, became the fastest selling book in history on the first weekend of its publication in 2000 - but its follow-up is set to break that record.
Rowling's Potter books have all been hugely successful and popular with both children and adults.
The first book, Harry Potter and the Philosopher's Stone was published in June 1997 to almost instant success.
And the first two books have also been made into box-office smash films, grossing more than £1bn worldwide.