Harry Potter author JK Rowling said how happy she was that Harry Potter And The Cursed Child would premiere without any spoilers leaking.
Arriving at the Palace Theatre in London's West End for the opening gala, she said she was delighted fans had kept the secrets of the new instalment in the series, adding: "They've been amazing, they've been incredible, and you know what it is?
"It is the most extraordinary fandom so I'm kind of not surprised they didn't want to spoil it for each other but I'm so happy we got here without ruining it."
Rowling added she was excited to see the play again as it had been a long time since she walked a red carpet at a Harry Potter premiere, and revealed she hopes the play will travel as widely as possible.
Asked if it will head to Broadway, she said: "I'd love it to go wider than that. I'd like as many Potter fans to see it as possible."
Mayor of London Sadiq Khan spoke of his pride that the play was launching in the capital.
He said: "Many thanks to JK Rowling for ensuring the premiere is here," and added that he was a "big fan" of Harry Potter, saying: "What's important is that the world premiere is here in London, and we should be really proud."
Director John Tiffany said he was excited to finally be officially opening the show after nearly eight weeks of previews.
He thanked fans for keeping the secrets so far, comparing sharing plot details with opening kids' Christmas presents in November: "Why would you do that?"
The Cursed Child opening in London will be immediately followed by the midnight release of the play's script book, allowing fans across the globe to find out what happens next to the boy wizard and his friends.
Set 19 years after the events of the seventh and final book, The Cursed Child brings back Harry Potter, now grown up and an employee at the Ministry of Magic.
Harry and his wife Ginny Weasley wave off their youngest son Albus Severus to their old wizarding school, Hogwarts - but once there, Albus struggles with the weight of his family legacy and goes to extreme and dangerous lengths to right the wrongs of the past.
Daily Telegraph critic Dominic Cavendish said that "British theatre hasn't known anything like it for decades".
The two-part play stretches over five hours and was co-devised by Rowling, written by Jack Thorne and directed by Tiffany.