Many sources say that this concert was interrupted due to a power cut - that's actually incorrect, power cut occured during one of the November shows at the Rainbow theatre. What happened during this March concert was that Brian broke his string during Liar (around the time when Freddie sings "Listen" and had to switch to his Fender Stratocaster backup guitar (there are pics to prove that).
Freddie Mercury (lead vocals, piano, tambourine),
Brian May (electric guitar, backing vocals),
Roger Taylor (drums, backing vocals),
John Deacon (bass guitar)