Star Wars Celebration Europe, held at ExCel London, is the first official Star Wars event ever to be held outside the US, according to organiser Lucasfilm.
A procession of Stormtroopers gathered outside the ExCel exhibition hall at London's Docklands to mark the start of the opening ceremony, which will be attended by actors Mark Hamill and Ian McDiarmid, who played Luke Skywalker and the evil Supreme Chancellor Palpatine respectively.
Steve Sansweet, Lucasfilm's head of fan relations, said: "This is shaping up to be one of the most extraordinary celebrations we've ever had."
The three-day event will include a Jedi Padawan training set, a Darth Vader hot air balloon, and an outdoor screening of Star Wars: Episode IV: A New Hope.
Fans will also be offered previews of The Force Unleashed, a new computer game, and the CG-animated television series The Clone Wars.