Ending months of hype, and elaborate measures to prevent details of the boy wizard's latest adventures leaking out, Harry Potter and the Half-Blood Prince hit the shelves at one minute past midnight London time.
Children from around the world descended on the Scottish city of Edinburgh, where Potter author J K Rowling began reading from the latest book as soon as the deadline passed.
In Sydney, Australia, about 300 Potter fans crammed into the city's largest bookshop waiting for secured boxes of books marked "embargoed" to be opened.
More than 1000 followers were aboard a special train called the Gleewarts Express, which took them to a secret location outside the city where they would receive their copies.
Train owner Roger Mackell refused to disclose their destination.
"They are going to another land. If I told you where, I'd have to kill you," he joked.
In London, Pottermania broke out with hundreds of parents and children queuing outside bookshops.
"Every book just gets bigger and bigger," said David Roche of Waterstone's book retailer, speaking hours before the launch.
"It's like a film premiere now but for a book, which is quite extraordinary. We had 2000 people queuing in the West End the last time a Harry Pottter book came out; we anticipate the queue is going to be even bigger this time."
The book's launch has triggered
Pottermania around the world
Staggering sales forecasts may explain why so much time and effort has gone into protecting the contents of the sixth and penultimate book in the Harry Potter series.
Waterstone's predicts the book will sell over 10 million copies worldwide in the first 24 hours. Around 275 million copies of the first five books in the series have been sold to date and three Harry Potter movies have grossed $2.5 billion.
With book sales likely to run into the tens of millions, when a handful of copies were inadvertently sold before the deadline in Canada, purchasers were ordered not to disclose its contents, and, according to media reports, even not to read it.
A website offering what it claimed was an electronic version of the book was closed down, and two British men were charged last month with firearms offences after allegedly trying to sell a stolen copy of the Harry Potter book to a tabloid newspaper.
Rowling first thought up the Harry Potter character in 1990, and after the original book Harry Potter and the Philosopher's Stone was turned down by several publishers, Bloomsbury finally offered to print it.
The adventures of Harry Potter and his friends at Hogwarts School of Wizardry and Witchcraft have won over a new generation of young readers and been adapted into a movie series.
They also made Rowling the wealthiest woman in the United Kingdom, with a personal fortune estimated in 2004 at $1 billion.