Around 13,000 people congregated to take part in the mobile phone operator's new commercial by singing hits such as The Beatles' Hey Jude and Britney Spears's Hit Me Baby One More Time. But the crowd got a shock when Pink broke into an impromptu rendition of her hit single So What from the middle of the mob.

Speaking after the event on Thursday evening, the 29-year-old singer said: "What a fantastic experience. "Everyone singing along together in the middle of London – it was incredible. I was thrilled to be a part of such an amazing event and to share it with so many people." The new advert, called Sing, is a sequel to T-Mobile's last television commercial, Dance, which featured 350 dancers performing a choreographed routine among commuters at London's Liverpool Street Station. Described as a "mass karaoke singalong", The latest offering was captured by handing out 2,000 microphones to members of the crowd, and was filmed by 24 cameras. During the event, compèred by presenter Vernon Kay, the crowd belted out versions of the Grease hit Summer Nights, Total Eclipse of the Heart, and I Say a Little Prayer. Pink joined in halfway through the crowd's rendition of the Erma Franklin hit Piece of my Heart.

Daniella Kostic, 18, from King's Cross, London, said: "It is amazing. We came down to film the advert but we did not expect anything like this."

Jamilla Gibbs, 63, from Clapham, South London, added: "We loved the first advert and really wanted to get involved in

the second. It's not dancing but we're still having fun."

T-Mobile spokeswoman Lysa Hardy said: "We wanted to get everyone to have a really good time. People really enjoyed the Dance campaign and we have had an amazing response to this one. "The first advert is a really tough act to follow." The previous T-Mobile advert has become an online sensation on YouTube, attracting more than 11 million hits. Fans organised a re-enactment of the advert in January at Liverpool Street Station, through the social networking website Facebook. However, police were forced to close the station due to safety concerns when it was swamped by around 12,000 people who turned up to dance in the "silent disco", listening to music through headphones.

The new advert premieres during the break of Britain's Got Talent on Saturday, May 2 at 9pm on ITV1.

Watch a fan's perspective of the event below: