Madonna paid tribute to Michael Jackson as she described him as one of the greatest performers the world has ever known.

A huge picture of Jackson as a child was beamed above the stage during a short dance melody dedicated to the star as the Queen of Pop performed at the O2 Arena in London.

One of her dancers, dressed in Jackson's trademark black hat, moonwalked across the stage to the delight of the huge crowd packed into the arena.

Short versions of two of Jackson's greatest hits, Billie Jean and Wanna Be Startin Somethin, were also performed.

Madonna then turned to the crowd and said: “To one of the greatest performers the world has ever known — Michael Jackson.”

Jackson had been due to perform 50 concerts at the O2 Arena starting later this month.

The star died on June 25 after suffering a suspected cardiac arrest.

Organisers of Michael Jackson's public memorial yesterday announced that British fans would be able to apply for tickets.

The clamour to attend the event in Los Angeles began as soon as details of how to register for the ballet were revealed.

British fans had been disappointed to read on the Staples Centre website that only US residents could apply for a chance of securing a ticket before the information was later changed.

A spokesman for the memorial's organiser said: “British fans can register for the memorial.”Other fans of the King of Pop will be able to watch Tuesday's event from their homes as organisers announced it will be broadcast around the world. The stadium's website temporarily crashed within moments of AEG Live president and chief executive Kim Leiweke announcing the opening of registration for the draw.

He said 17,500 free tickets will be available in pairs for Jackson fans, 11,000 of which will admit entrance to the Staples Centre itself with 6,500 places to watch on screens at the nearby Nokia Theatre.

No information was given about what the memorial service would involve or which celebrities may appear.

Jackson's legion of fans has been descending on Los Angeles since his sudden death at the age of 50.

On Thursday a 30-second snippet of the singer rehearsing two days before his death was released as part of more than 100 hours of footage that could be turned into live albums, a movie and a pay-per-view special. The material along with possible insurance proceeds and ticket sales to memorabilia collectors could help benefit Jackson's estate which is burdened by an estimated US400million in debt.

\[Brian Lovett\]Details emerged as a law enforcement official said the powerful sedative Diprivan was found in Jackson's home. Diprivan is an anaesthetic widely used in operating theatres to induce unconsciousness. Also known as Propofol it is administered intravenously and is very unusual to have in a private home.

Footage of the singer rehearsing at the Staples Centre two days earlier was released on Thursday.

The star appeared in full flow as he danced around the stage in typical fashion to some of his most famous hits for the final time.

Kevin Mazur, 48, the photographer who captured some of Jackson's final moments on camera at the rehearsals said the star appeared to be back to his best and had “incredible energy”.

Mr Mazur, co-founder of WireImage, told the Daily Mirror: “When he hit that stage everyone was just mesmerised.”

It also emerged on Thursday that America's anti-drugs agency had launched an investigation into Jackson's death.

The Drug Enforcement Administration (DEA) was called in by Los Angeles police to assist as they probe allegations that the star was on powerful painkillers, sedatives and anti-depressants, sources close to the investigation said.