Harry Potter made his stage debut on Tuesday in a new London play that imagines the fictional boy wizard as a father of three, in the latest offshoot of the globally successful franchise.

Harry Potter and the Cursed Child is set 19 years after the seventh and final book in the series by J.K. Rowling, which have sold more than 450 million copies since 1997 and been adapted into eight films.

Like many of his fans, Potter has now grown up and has three children with his wife Ginny Weasley, the sister of his friend Ron, and is working at the Ministry of Magic.

He still has his trademark round-rimmed glasses and the scar on his head, a permanent reminder of his nemesis Lord Voldemort, but must now help his youngest son Albus confront the family's dark past.

Cut-price previews for the play at the Palace Theatre, in London's West End, began on Tuesday ahead of the world premiere on July 30.

Rowling pleaded with spectators at the previews not to disclose the details of the play.

"You've been amazing for years at keeping Harry Potter secrets so you didn't spoil the books for readers who came after you," she said.

"So I'm asking you one more time to keep the secrets and let audiences enjoy Cursed Child with all the surprises that we've got to the story."

The anticipation has been building for months, not least among its stars, including Jamie Parker, the 36-year-old actor who plays Potter.

"These are stories that people have lived with for their whole lives, they've grown up with and now they're adults rejoining the tale, picking it up where they left off - and I'm one of them," he told the official Pottermore website.

The play looks set to be a commercial hit. The first 175,000 tickets sold within 24 hours in October, and the script is already a best-seller before it has even been published.

Some fans queried the casting of a black actress, Olivier award winner Noma Dumezweni, as Potter's sidekick Hermione Granger, played in the films by Emma Watson.

But Rowling backed the actress, saying Hermione was described as "brown eyes, frizzy hair and very clever. White skin was never specified".