She met up with co-stars Daniel Radcliffe and Rupert Grint and Potter creator J.K. Rowling, and, after meeting fans, signing autographs and posing for photographs on the red carpet, it was clear that both Watson and Rowling were overcome with emotion.
The sobbing author told fans, "Thank you for queueing up for the books for all those years, for camping out in a wet Trafalgar Square."
Meanwhile, Watson struggled to hold back tears as she attentively listened to Radcliffe and Grint talk to reporters and TV crews about being part of the decade-long film franchise.
She was comforted by producer David Heyman, who planted a kiss on her forehead as she cried on his arm. She was later photographed resting her head on Grint's shoulder as she fought back tears.
The actress looked on wistfully as Radcliffe, who had flown in from New York for the premiere, addressed the crowd, saying, "I don't think the end of the story happens tonight because each and every person who will see this film will carry this story with them through the rest of their lives.
"A huge thank you to all of you, first of all for giving me a job... Every opportunity I get from now on all goes down to the fact I got very, very lucky when I was 11."
But an emotional Watson shrugged off her co-star's modesty, telling him, "You didn't 'get lucky - you were, and are, the perfect Harry, and will be forever."