Saatchi & Saatchi will create an ad from the footage from the gathering.
The TV commerical will be edited in 48 hours and will premiere on 2 May during the 9pm break of Britain's Got Talent on ITV1, and simultaneously on YouTube at .
It follows the mobile phone company's flash-mob ‘Dance' ad that took place at London's Liverpool street in January which has been viewed on YouTube more than 11m times.
Lysa Hardy, head of brand and communications at T-Mobile, said: ‘"Dance" was always going to be a hard act to follow, but we think we've created a worthy sequel with ‘Sing-along'."
Yesterday's event, hosted by DJ Vernon Kay, attracted around 13,000 people. They congregated from 6pm and participated in a mass karaoke singing renditions of The Beatles' Hey Jude and Britney Spears' Hit Me Baby One More Time.
Camera crews mingled with the crowd to catch them on a large screen before Pink appeared and performed her hit single So What.
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