Fans gather in London where Jackson was to perform

LONDON (Reuters) - Hundreds of Michael Jackson fans gathered outside London's O2 Arena on Monday to pay their respects to the entertainer on the day he was to have opened a run of 50 concerts at the venue.

Jackson died on June 25, just weeks ahead of his eagerly awaited comeback following years of living as a virtual recluse.

The London concerts sold out in record time and some of the 750,000 tickets sold traded hands at hugely inflated prices on the Internet.

Underlining fans' devotion to the "King of Pop", hundreds turned up to the venue anyway, determined to celebrate their favorite musician.

"This was going to be the best summer of my life," said fan Michael. "I was going to go to five shows altogether, it would have been fantastic. I can't believe rather than coming here to see him perform I'm here to mourn ... It's devastating, it really is."

Some fans sat quietly beneath a large screen with a picture of Jackson and the words "Michael Jackson 1958-2009" reflecting on what might have been.

Others, including several Jackson impersonators, led sing-a-longs of his greatest hits.

Will, another fan, said: "We've come here today even if there's no gig tonight. We're making our own gig, our own carnival atmosphere, we're making this what it would have been if Michael had been here."

The man behind chart-topping songs like "Thriller," "Billie Jean" and "Man in the Mirror" has been mourned the world over since his sudden death at the age of 50, and sales of his records have soared.

AEG Live, promoters of Jackson's planned comeback "residency" in London, have offered fans either full refunds for the canceled shows or the option of keeping their commemorative tickets as keepsakes.

Jackson died while rehearsing for the concerts, and AEG has said it hopes to stage a tribute including elements of the This Is It spectacle. Several reports have said organizers are aiming for a date in late August.

(Reporting by Mirja Spernal; writing by Mike Collett-White; Editing by Jill Serjeant)