A group of protesters held up distinctive yellow placards with the message "Not my king", joining a crowd waving Union Jack flags.
The demonstration was organised by "Republic", a group calling for an end to the monarchy and an elected head of state in Britain.
Local newspaper the Milton Keynes Citizen reported that Charles "ignored the small banner-waving group" of 20 or so protesters.
Comment:
Republic's leader Graham Smith wrote on Twitter: "I asked Charles why he's wasting money on the coronation. He didn't want to answer."
"We're determined to get the message across that it's OK to protest against the royals," he added.
Republic has announced it will hold a protest at the coronation on May 6.
The death of Elizabeth II in September has prompted questions over the future of the royals under Charles and his queen consort Camilla, as the family has been rocked by criticism from Prince Harry and Meghan Markle and sex crime allegations against Prince Andrew.
Comment: As well as King Charles' own links with known paedophiles: The Prince and the pedophile: Charles' connections to pedophilia networks
Since the queen's death, several anti-royal protesters have been arrested for holding solo pickets, after legislation on protests was made tougher.
Charles was attending a reception at a church to mark Milton Keynes becoming a city as part of celebrations for Queen Elizabeth II's Platinum Jubilee.
He also visited a food bank.
And his ability to rule the Nation. This is all out in the open, in the tabloid press.
From the Spider memos, where he attempted to influence parliamentary issues,
his influence in armaments sales to the petrochemical producers of the middle east to buy British armaments. His very dubious, connection with Jimmy Saville.
His longstanding relationship, even when married to Diana. And the issues surrounding her death. And let us not forget his WEF connections with Claus Schawb, and the "Great Reset".
Is it any wonder he is being pelted with eggs, and the people are protesting, they have no respect for him as a leader of the country.
I suspect, we will see him abdicate in favor of Prince William, but I wonder, will this be a case of like father, like Son. A photogenic happy family couple, all photo ops highlighting social issues, in essence platitudes, from a highly privileged family, but in reality, the public knows nothing about them.