Around 600 people gathered outside the 23,000 seat arena, many of them bearing candles and flowers. Some were even wearing the singer's signature look of a fedora hat and one white glove.
At 6.30pm that evening – the time the doors should have been opening for Michael's first This Is It concert - the crowd held a minute's silence.
While for many it was an emotional experience, the evening has also been described as having a "celebratory" mood.
Michael sing-a-longs and impromptu dance performances took place, while fans signed a special memorial wall dedicated to their hero, who passed away on June 25. Video clips of the singer in his heyday, as well as footage from his last breathtaking rehearsal for his O2 residency, were also broadcast on big TV screens.
"It’s a way of getting closure," one British fan said of the evening's gathering. "I cried when I signed the memorial wall, but apart from that the atmosphere has been happy. London has become the focal point for all his fans in Europe because of the concerts, so it was important to be a part of this."
Following the star-studded memorial concert held in Los Angeles last week, a one-off tribute gig at the O2 is expected to be announced within days. Justin Timberlake, Leona Lewis and Usher are reportedly among the performers who have been approached to take part in the concert, thought to be taking place on August 29 – which would have been Michael's 51st birthday.