London, UK - 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 hospitalization and its postponement.

The 65-year-old 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.

Madonna may have put her 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.

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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 favorites 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.

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.

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

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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.

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.