The seventh and last novel in the Harry Potter series is set to be confirmed as the world's fastest-selling book.
Fans around the world queued to get the book overnight on Friday
In the US, an estimated 8.3 million copies were sold in the first 24 hours, according to publisher Scholastic.
That beats the previous record, held by the sixth Potter book, which sold 6.9 million in its first day in the US.
UK sales figures are due later, but WH Smith said it sold 15 copies of Harry Potter and the Deathly Hallows per second when it came out on Saturday.
A spokeswoman at Bloomsbury, the book's British publisher, told the Observer newspaper that sales of the final instalment could reach three million copies in the first 24 hours, up from two million for 2005's Harry Potter and the Half-Blood Prince.
UK supermarket chain Asda also confirmed that Deathly Hallows was selling twice as quickly as its predecessor.
The store, which discounted the book from £17.99 to £5, said it shifted 97% of its 500,000 copies in the first 24 hours.
"The nation really has gone Potter potty," said spokesman Ed Watson.
Fans dressed up as characters from the novels for the UK launch
After weeks of hype, the seventh Harry Potter book finally hit shop shelves on Saturday.
It was released at midnight BST, meaning that in countries like India and Australia, fans were waking up to magic breakfasts laid on by book shops.
In Bangladesh, customs offices continued to work on a Friday - a holiday in the country - to ensure the novel was delivered on time.
In the UK, 250,000 fans - many dressed as wizards and witches - turned up to midnight launch parties hosted by book store Waterstone's.
Fans in the US had to wait slightly longer, as booksellers observed their own midnight embargo, but they still turned out in their thousands to buy the book.
The Borders book chain said 800,000 devotees attended the countdowns in their 1,200 US stores, with first day sales reaching 1.2 million.
But while sales records have been smashed, smaller booksellers said the Potter phenomenon was "irrelevant" to their business.
"The only copies we sell are to people who are our regulars," said Jonathan Main of the independently-run Booksellers Crow in Crystal Palace, London.
"The passing trade aren't going to come here and buy it."
"We'll sell 90 copies over the weekend, then we won't sell another copy for a year," agreed Johnny de Falbe, manager of the John Sandoe bookshop in Chelsea, London.
"I like Harry Potter, but it's a shambles as far as the retailing trade is concerned because nobody makes any profit."