Daniel Radcliffe, who plays the film's title character, had his stylish gray suit and pink shirt drenched in the rain, despite efforts from a bodyguard holding an umbrella to keep the water away.
Still, the actor smiled and greeted fans that numbered in the thousands to see him, the Telegraph reported.
"Frankly, I think they would turn out in earthquakes and tornadoes," actor Jason Isaacs, who plays Lucius Malfoy, told the British newspaper. "That's the effect the 'Harry Potter' stories have."
Rupert Grint, who plays Potter's pal Ron Weasley, had recovered enough from what his publicist previously told the BBC was a "mild" bout of swine flu to attend the event. He too, however, was soaked in the rain.
Things faired better for Emma Watson, Hermoine Granger, in the series, whose vintage dress from '60s designer Ossie Clark, dried quickly. Unfortunately, as the 19-year-old actress shook out her dress, Emma gave photographers a quick peek of her underwear.
Other stars in attendance at the London event included series author, J.K. Rowling, Helena Bonham Carter, who plays Bellatrix Lestrange, Alan Rickman who stars as Severus Snape and Tom Felton who plays Draco Malfoy.
U.K. late night host and funnyman Graham Norton turned up as did former "Life" actor Damian Lewis, celebrity chef Jamie Oliver and Guy Ritchie who brought his son with Madonna, Rocco.
"Harry Potter and the Half Blood Prince" opens stateside on July 15.
Related Content from AccessHollywood.com:
PLAY IT NOW: Preview: 'Harry Potter And The Half-Blood Prince'
PLAY IT NOW: ACCESS EXTENDED: 'Harry' Gets Handprints