The performance took place at an advanced screening of the Salman Khan and Kareena Kapoor film at Cineworld Haymarket.
Ashanti Omkar, Bollywood editor for BBC Publishing, told Digital Spy: "There was much excitement about the special preview screening of Bodyguard, as Ash King, the singer of the chart topping 'I Love You', was spotted arriving."
The song has had a combined total of more than 1.5 million hits on YouTube since its release, and has also maintained a position in the Top 10 on the iTunes World Music chart.
"With a lot of screams from his fans, he spoke about working with music director Pritam and his excitement to be a part of this year's Eid blockbuster," added Omkar. "He then sang a line or two from the song, dazzling the audience into a frenzy, just in time for the film to start."
Trade analyst Taran Adarsh has indicated that the action film will break current Indian cinema box office records.
"Bodyguard has monstrous, earth-shattering, mind-boggling opening," posted Adarsh on his Twitter account.
King is scheduled to sing his current track, as well as 'Dil Gira Dafatan' from Dehli 6 and 'Suno Aisha' from Aisha, at the London Mela on September 4.
> Read Digital Spy's Salman Khan interview here
Watch an acoustic version of 'I Love You' below: