The King joked that he is "still alive" as he and the Queen stepped out for their final public engagement before their Christmas break.
Charles, 76, and Camilla, 77, attended a reception in celebration of community cohesion at Waltham Forest Town Hall in east London on Friday.
Asked "How are you?" by Sikh faith representative Harvinder Rattan, the King smiled as he replied: "I'm still alive."
The monarch's quip came hours after Buckingham Palace sources told Sky News that "his treatment has been moving in a positive direction and as a managed condition the treatment cycle will continue into next year".
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© GettyDelivering presents
During the visit, Camilla donated 25 toys to Citizens UK, to be passed on to children living in asylum hotels, and a donation to a food bank was left on the King’s behalf.
The donation included Waitrose cartons of long-life milk, Christmas puddings, custard and mince pies.
George Gaillett, community organiser at Citizens UK, said afterwards: "The Queen said, 'We hope you can distribute this and they can go to the right people'."
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© GettySeasons greetings
After listening to a performance by a children's choir, A Little Choir of Joy, Charles told them: "I do hope you have a very happy Christmas."
He then joked that the children in the choir, made up of eight to 12 year-olds from schools in the borough, should "make the most of it" for the "poor teachers".
Charles was presented with a Christmas card from the choir, with messages written inside which included "Your Majesties, thank you so much for visiting us today" and "hope you enjoyed our singing".
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© GettyWalk down memory lane
During the walkabout, the King also met Caroline Akuffo, 77, who showed him a black and white photo she had kept framed on her wall from a meeting in Osaka, Japan, in 1970.
She said: "It was a trade fair. He came to the pavilion and I remember shaking his hands and they were so soft it was like holding cotton wool.
"He was so happy to see the photo and said I was wearing a lovely dress."
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© GettyBorough of Sanctuary
The King and Queen's visit to Walthamstow comes just months after thousands of people gathered in Walthamstow for a counter-demonstration on August 7, and those planning to protest outside an immigration centre failed to turn up.
The area in north-east London was one of hundreds of locations earmarked for protests, and there were fears of widespread unrest.
Instead, at least 5,000 counter-protesters gathered outside the immigration centre, taking a stand, they said, against racism and violence.
Buckingham Palace said August's peaceful protest demonstrated the "true community spirit of the borough, coming together to keep people and communities safe".
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© ZUMA Press Wire/ShutterstockChristmas break
Charles and Camilla are expected to travel to Sandringham in the coming days, where they will host members of the royal family for Christmas.
