The publisher of Prince Harry's controversial memoir says sales have exceeded its "most bullish expectations".
Transworld Penguin Random House said Spare was its fastest-selling non-fiction book with sales so far topping 400,000 copies.
That includes hardback, audiobook and ebook downloads.
Harry Potter and the Deathly Hallows sold 1.84m units in its first week in 2007 – still a UK record for fiction.
Transworld managing director Larry Finlay said: "We always knew this book would fly but it is exceeding even our most bullish expectations.
"As far as we know, the only books to have sold more in their first day are those starring the other Harry (Potter)."
Some bookshops in the UK opened hours early overnight on Tuesday in anticipation of strong sales, with industry experts predicting that Spare could be one of the best-selling pre-order titles of the past decade.
People queued for midnight shop openings, with a handful waiting outside WH Smith in London's Victoria station to be among the first to buy a copy.
The 407-page book sees Harry expressing frustration at being "spare to the heir", anger at sections of the UK media, revealing unresolved trauma over the death of his mother, his struggles with mental health, the isolated life he led before meeting Meghan, and the breakdown of relations with the Royal Family.
It is being sold for less than the recommended retail price in outlets such as Waterstones and WH Smith as well as online at Amazon.
US retailer Barnes and Noble said the pre-publication leaking of details from the book had "only heightened the frenzy".
Shannon DeVito, a director of the American-based chain, said: "It is thrilling as a bookseller to have a massive release like this to start our new year.
"The leaking of extracts has only heightened the frenzy and is drawing customers into our bookstores in amazing numbers."
Prince Harry has been criticised over a number of frank admissions in Spare, including that he killed 25 Taliban fighters during his time in the British Army in Afghanistan.
In the text, ghostwritten by Pulitzer Prize winner JR Moehringer, he described the killings as like "chess pieces taken off the board".
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