The long-anticipated book "The Casual Vacancy" drew lines in front of British bookshops, which wheeled out crates of the books precisely at 8 a.m.
Bookmaker William Hill put 2/1 odds on the new work outselling "Harry Potter and the Deathly Hallows," which shifted 2.6 million copies in Britain on its first day.
Many of the early buyers were Harry Potter fans who, like the author, have moved on to more adult fare.
The book, which is Rowling's first full-length work since the final Potter book five years ago, concerns the death of a local government leader. Rowling has said she was aiming for Dickensian sweep in the multi-character saga, whose doses of sex, satire and swearing mark a distinct departure from wholesome Harry.
"Vacancy" is receiving mixed reviews from critics. It's likely nothing Rowling publishes will ever match the success of the Potter books, which have sold more than 450 million copies around the world.
The Associated Press contributed to this report.