It marked the first time that a new season of the HBO series has gotten its world premiere in Europe.
Sky Atlantic, the network of pan-European pay TV giant Sky, hosted the red-carpet gala in the British capital, said The Hollywood Reporter.
Sky is HBO's pay TV partner in Western Europe and airs "Game of Thrones" across its markets in the UK, Ireland, Germany, Austria and Italy.
Among the stars turning out for the event and after-party were Kit Harington (Jon Snow), Nikolaj Coster-Waldau (Jaime Lannister), Natalie Dormer (Margaery Tyrell) and Mark Gattis (Tycho Nestoris), who is also a writer and star of "Sherlock".
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Plepler and Darroch spoke before the screening of the season 5 opener, with both touting Game of Thrones as "a global phenomenon."
Show creators David Benioff and Dan Weiss, who missed the premiere to complete the work on the season, recorded a video message for the fans.
Benioff said the two were stuck in LA but hoped everyone would enjoy the season premiere and the event. Weiss called the Tower of London "the Throniest location" the two could imagine and warned the audience to "keep your hands off the Crown Jewels," which are on display at the Tower.