The son of Yoko Ono and John Lennon, Sean Lennon, has opened up about his terror over his mother's safety whilst she was on stage with Lady Gaga last month.

The duo performed together during a gala concert at the Orpheum Theatre as a tribute to John Lennon in the week leading up to what would have been his 70th birthday. Sean was the musical director for the event. 

Lennon said: "Gaga was killing it as always and everyone was standing on their feet and screaming. And Yoko was looking at this and she was like, 'Hmm ...' She was not to be outdone! So she climbed all over the piano - and she's 77! And Gaga climbed up after her. They both started rolling around and singing upside down on top of the piano. It was amazing."

He continued: "I was proud but I was little bit scared, too. It was a grand piano and it was slippery and they were both in heels. They were both wearing fine silk fabrics. I just didn't know what was going to happen. Should I catch her or should I cue the end of the song? What could happen now, other than something terrible?"

Lennon quickly gave the rest of the musicians the signal: 'OK, let's just end it,' and the performance ended.

Other guest performers at the tribute concert included Iggy Pop, Paul Simon, Vincent Gallo and Wu-Tang Clan's RZA. Yoko Ono performed with her conceptual group, the Plastic Ono Band.

Watch Yoko Ono and Lady Gaga performing 'The Sun is Down' at the concert below: