The King told a royal fan he is “doing his best” after he attended the Easter Sunday service at Windsor Castle.
Charles shook hands and spoke with crowds who had gathered to wish him well during his first major public appearance since revealing his cancer diagnosis to the world.
When one fan shook his hand and told him to “get well soon”, Charles responded: “I'm doing my best.”
Others shouted out: "Keep going strong, never give in."
Another told him: “We’re all rooting for you, we’ve all got your back.”
The King shook hands with multiple people after he and Camila re-emerged from St George’s Chapel to cries of “Happy Easter”, before a young boy presented Camilla with a posy of white and yellow flowers.
Charles had smiled and waved at crowds as he arrived for the Easter Mattins Service alongside the Queen, joining other members of the royal family – including the Princess Royal and her husband Vice Admiral Sir Timothy Laurence, the Duke and Duchess of Edinburgh, the Duke of York and Sarah, Duchess of York – for the annual event.
He spotted Anne Daley, 65, from Cardiff who had travelled down by train to catch a glimpse of the His Majesty and was waving a Welsh flag.
Gesturing to Camilla, he said: “Look, she’s from Wales.”
Ms Daley, a committed royalist, said afterwards: “I shouted back ‘yes, I’m from Wales and have come here especially to see you’. He looked absolutely marvellous, really well.’
She added: “I wanted to show my support for King Charles and the Prinfcess of Wales who we are all wishing well. We want to show them that we miss them, Wales misses her. She may not be here but she is not forgotten.
A Palace source said the event was a “significant step” since the King stepped away from public life almost eight weeks ago - although he has kept up with his duties as head of state.
The source was quoted by The Sun as saying: “As can be seen, The King has responded to treatment very encouragingly over the past weeks and his doctors were thus able to adjust their guidance slightly on what His Majesty is now able to undertake, including attendance at the Easter Service and greeting well-wishers who had kindly turned out to show their support.
"To be clear, His Majesty's treatment continues and caution is of course the watchword, but as diary plans are evolved towards summer, we hope to see more of these carefully-calibrated steps towards the resumption of some public facing duties for The King, with adjustments made where necessary.
"Nothing will be confirmed until nearer the time, plans remain in flux and are subject to medical guidance, but there is great hope and optimism from both doctors and the patient.”
It comes just over a week after the Princess of Wales released an emotional video message disclosing that she had started a course of preventative chemotherapy.
Kate, the Prince of Wales and their children Prince George, Princess Charlotte and Prince Louis, who all attended the service last year, were absent from the Easter Sunday service.
The family are spending the Easter holidays together as they adjust to Kate’s diagnosis, which was discovered in post-operative tests after major abdominal surgery.
The King’s attendance at church will be seen as a move to reassure the public after the shock news about his daughter-in-law.
But the service was a smaller version of the annual gathering, with fewer members of the royal family, as the King has paused public-facing duties while he continues treatment for cancer himself.
As is customary, the King sat inside the chapel in the Sovereign’s Seat which is situated at the front of the altar, along with his family who were in their usual places in the stalls.
The layout of St George’s Chapel is such that the main congregation sits in a separate section of the church.
He was described by the palace as being “so proud” of the princess for her courage in speaking out, and is said to be in “the closest contact with his beloved daughter-in-law”.
The King’s own treatment for cancer was announced at the start of February, but he has been carrying out low-key official duties behind palace walls.
Towards the end of February, Charles had an audience with Prime Minister Rishi Sunak and held a Privy Council in his first face-to-face official duties since the diagnosis.
Earlier in March, the King held an in-person pre-Budget audience with Chancellor Jeremy Hunt, had a video call with Canadian prime minister Justin Trudeau, and had an audience with Baroness Scotland.
Over the course of the month, he also welcomed a series of foreign officials to Buckingham Palace, including high commissioners of Jamaica, Tanzania and Singapore.
On Tuesday, as the palace announced Charles’s attendance at the Easter Sunday service, he was pictured carrying out official duties for the first time since Kate shared her cancer news with the nation, as he met community and faith leaders from across the UK in London.
Ahead of Easter, the King reaffirmed his coronation pledge “not to be served, but to serve” with “my whole heart” in an audio address broadcast to a congregation at Worcester Cathedral where the Royal Maundy Service was held in his absence on Thursday.
