Charles, 75, appeared to be in good spirts and had a smile on his face as he waved to admirers.
It marked the first time he's been seen since the news broke on Monday, February 5.
As OK! reported, an insider claimed the patriarch's prognosis is "good."
British Prime Minister Rishi Sunak emphasized that during an interview with BBC Radio 5 Live.
"Obviously, like everyone else, shocked and sad, and just all our thoughts are with him and his family," he replied when asked how he felt upon hearing the news. "Thankfully, this has been caught early."
Sunak added that he hopes Charles "gets the treatment that he needs and makes a full recovery."
"That’s what we’re all hoping and praying for, and I’m of course in regular contact with him and will continue to communicate with him as normal. He’ll be in our thoughts and our prayers, many families around the country listening to this will have been touched by the same thing and they know what it means to everyone," he continued. "So we’ll just be willing him on and hopefully we can get through this as quickly as possible."
Though Charles will be taking a step back from some of his royal duties, Camilla, 76, is set to carry on with her engagements.
In addition, it's been said that Prince William may take on some of his father's responsibilities.
An insider revealed Charles informed both of his sons, as well as other family members, about his condition before Buckingham Palace made the official announcement.
"During The King's recent hospital procedure for benign prostate enlargement, a separate issue of concern was noted. Subsequent diagnostic tests have identified a form of cancer," the public message began.
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"His Majesty has today commenced a schedule of regular treatments, during which time he has been advised by doctors to postpone public-facing duties," the note continued. "Throughout this period, His Majesty will continue to undertake State business and official paperwork as usual."
"The King is grateful to his medical team for their swift intervention, which was made possible thanks to his recent hospital procedure. He remains wholly positive about his treatment and looks forward to returning to full public duty as soon as possible," the statement concluded. "His Majesty has chosen to share his diagnosis to prevent speculation and in the hope it may assist public understanding for all those around the world who are affected by cancer."