Pop legend Madonna kicked off her 40th anniversary "Celebration Tour" in London on Saturday, enthralling fans three-and-a-half months after a bacterial infection led to her hospitalisation and its postponement.

The 65-year-old US superstar spent several days in intensive care in New York after being found unconscious in her apartment there in June, and has since been recovering, and rehearsing, for the rearranged 78-date tour.

She had not been due to begin at The O2 Arena in the British capital, but after postponing the original July 15 start date in Canada, it became the curtain raiser -- to the delight of fans in the UK. 

"I wasn't expecting it to be the opening night but as things are, it is really, really excited to see her," teacher Kate Taylor, 46, told AFP ahead of the concert.

"I'm really excited for the greatest hits because I think that's what we all want to see," she added.

Janet Hutton, turning 50 on Sunday, had travelled from Slovakia to see her icon for the fourth time.

"Madonna is the queen of pop," she said. "You have other people, like Britney or Beyonce or Ariana Grande, but there's only one Madonna. 

"And when you're a big fan of Madonna, you feel that you live with her and we live with her now and we're very excited to see her."

- Tech issues -

Madonna may have put health woes behind her, but she was also forced to overcome some early on-stage technical hitches in the first of four sold-out performances at The O2.

After taking to the stage in spectacular fashion -- with a powerful rendition of her 1998 hit "Nothing Really Matters" -- she had to stop the show briefly to "press the reset button" following sound issues.

"This is exactly what you don't want to happen on your opening night so this wasn't planned. I'm sorry," Madonna told the crowd.

After the problems were resolved, she launched into lively versions of 1983 breakthrough single "Holiday", 2005's "Hung Up" and other favourites, while running through an array of elaborate costumes.

They included a punk rocker-style ensemble featuring a classic statement bra, black jacket and chains, for her 1982 track "Everybody".

For the first time since her early days, Madonna will not be performing alongside an on-stage band during the tour.

But her daughter Mercy James made a special surprise appearance on opening night, to play a piano on stage as her mum sang her track "Bad Girl".

Madonna paused her show again later, this time to send a message of support for Israelis and Palestinians "suffering" amid the "heartbreaking" violence there.

In other messaging during the hours-long gig, a photograph of the late singer Sinead O'Connor appeared on large screens while Madonna draped a Ukraine flag over her back before launching into "Don't Cry for Me Argentina".

- Committed -

Madonna -- whose full name is Madonna Louise Ciccone -- started out as a dancer in the late 1970s in New York.

Thanks to classics including "Like A Virgin" and "Material Girl", she went on to win seven Grammy Awards and sell more than 300 million records worldwide.

Over the years she has also taken in acting, film directing and business ventures.

Her 40th anniversary tour will also include shows in Paris in mid-November, in Montreal in January, before winding up in Mexico on April 24.

The tour promises to be a "documentary through her vast career" drawing on archive footage and studio recordings from her four decades in the spotlight, according to her musical director Stuart Price.

"A greatest hit doesn't have to be a song. It can be a wardrobe, it can be a video, or a statement," Price told the BBC in a recent interview.

The show's set-list, which slowly emerged Saturday night, had been kept a closely guarded secret.

Shows are set to feature around 45 songs in total, 25 in their entirety and extracts of around 20 more.

The singer known for her sometimes provocative outfits -- such as the pink conical bra outfit by Jean Paul Gaultier -- will be dressed by the Georgian Guram Gvasalia, director of the young label Vetements.

Her commitment to the tour was underlined in July, when she posted on social media about her hospitalisation.

"My first thought when I woke up in the hospital was my children," she wrote. 

"My second thought was that I did not want to disappoint anyone who bought tickets for my tour." 

but-jj/jj