Bloomsbury, the publisher, estimated three million copies were sold in the first 24 hours, almost a million more than the last Potter book.
W H Smith and Woolworths, which have the book for 6.99 pounds, said it was selling at more than 15 a second.
Waterstone's, whose flagship Piccadilly store was the focus for the capital's celebrations, welcomed 250,000 visitors in the hours immediately after the launch and sold more than 100,000 copies in two hours.
By Saturday afternoon, it was selling 40,000 copies an hour.
Asda, which has the book for 5 pounds, less than a third of the price recommended by the publisher, sold its entire stock of 500,000 copies on Saturday.
The frenzy was sparked by fans' determination to discover the character's fate first-hand rather than have the story spoilt for them.
Many Harry Potter fans in London did not return home after buying the book, choosing to read it in cafes or the street.
Fiona Allen of Waterstone's said "They just couldn't wait a moment longer. The statue of Eros in Piccadilly was awash with people reading the book."
The scenes in Britain were repeated around the world as fans queued outside shops in New York, Los Angeles, Paris, Rome and Sydney. - DDNEWS India
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