The King was applauded onto the Royal Opera House stage as he attended a gala celebrating the conductor at his coronation.
The event paid tribute to Sir Antonio Pappano at the end of his 22-year tenure as music director of The Royal Opera.
Performers were told Charles would be attending 20 minutes before they went on stage, and their show closed to a standing ovation.
Asked how he was feeling after the show, Charles, who has been receiving treatment for cancer, replied: “Not too bad.”
After the curtain fell Charles and Sir Antonio spoke about the time that has passed since the coronation last May.
Sir Antonio said: “He was just so generous in his praise, and genuinely so, and that means everything, he loves music.
“He knighted me, and I’ve done several events at Buckingham Palace over the years where he’s been the engine for those things, and of course I conducted at the coronation, so we’ve done quite a few events.”
He said about the gala: “These kinds of programmes are almost impossible, with so many different pieces from one to the other, but the feeling that everybody wanted to be here, we had a full house, the King was here.”
Opera tenor Freddie De Tommaso said he and other performers were told Charles would be there 20 minutes before going on stage.
He said: “It was one of the most exciting events I’ve ever been a part of.
“Just to have this group of about 15, singing with these colleagues, and friends now, is just amazing, and all here because we all love Tony so much and he’s inspired intense joy and happiness.
“It’s amazing the King was here, they told us about 20 minutes before, ‘right, just so you’re aware, there’s a special guest’.
“I was like oh my god, cool, great. He just said he really enjoyed it, what an honour, to sing for a king, doesn’t get any better than that does it, I’m chuffed I must say.”
Mezzo-soprano opera singer Aigul Akhmetshina, from the Republic of Bashkortostan in Russia, said “it was better than I expected” and that Sir Antonio “is her guiding light”.
“I kind of expected (the King) might be here, but it’s so amazing that he found the time to come and support.
“He just said I was very good at acting and to keep going.”
After conducting at the coronation Sir Antonio was made Commander of the Royal Victorian Order in this year’s New Year Honours, and won his second Olivier Award for outstanding achievement in opera.
He was knighted for his services to music in 2012 and in the same year made a Cavaliere di Gran Croce of the Republic of Italy.
Off the stage he has presented the BBC documentaries Opera Italia, Pappano’s Essential Ring Cycle and Pappano’s Classical Voices.
From 2005 to 2023 he was music director of Rome’s Orchestra dell’Accademia Nazionale di Santa Cecilia.
The King’s comment about his health came after the Queen said he is “getting better”, following a decision to granted permission for him to attend more public engagements.
Speaking to guests at a garden party at Lamb House in East Sussex on Thursday, Camilla joked that Charles “would be (getting better) if he behaved himself”, after he attended five event in just 48 hours this week.
He visited a Hampshire airbase to hand over a role with the Army Air Corps to the Prince of Wales, hosted his first investiture ceremony since returning to official duties, unveiled his first portrait as king, attended a service honouring the Order of the British Empire at St Paul’s cathedral and hosted a garden party for 4,000 people from the creative industries.
