The King has received more than 7,000 letters and cards from across the world since his diagnosis earlier this month, the palace revealed.
Many came from children and included hand-drawn images, while others were from cancer survivors sharing their personal battles, it added, thanking “everyone who took the time to write.”
“Such kind thoughts are the greatest comfort and encouragement,” Charles said.
The British monarch also said the messages had “reduced me to tears most of the time.”
Buckingham Palace announced on February 6 that the 75-year-old British King had been diagnosed with an undisclosed form of cancer and would be stepping back from public-facing duties while he undergoes treatment.
It was identified after he was admitted to a London hospital for a corrective procedure for an enlarged prostate last month, though prostate cancer has been ruled out to CNN by a royal source, who did not provide any further detail.
Charles was seen back at work for the first time on Wednesday, as he was filmed shaking hands with British Prime Minister Rishi Sunak during a meeting that marked the resumption of their weekly encounters.
Though the King has stopped carrying out public engagements while he undergoes his cancer treatment, he is continuing with his duties as head of state.
Alongside the statement of appreciation, Buckingham Palace on Friday also shared some of the images and videos the king has received.
One such message that provoked “much royal mirth” was a card from an adult featuring a drawing of a dog wearing a veterinary cone, with the caption “King Charles, at least you don’t have to wear a cone,” Buckingham Palace said.
Another message said, “I wanted to tell you I am thinking of you as you face your own diagnosis and treatment and send prayers and every good wish for a speedy recovery.”
“Chin up, chest out, remain positive and don’t let it get you down. Trust me, it works, but the main thing is family.”
“Never give up. Be brave. Don’t push your limits. Get well soon,” read another.
