King Charles III on Thursday encountered a group of anti-monarchy activists while on walkabout in the city of Milton Keynes north of London.
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.
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.
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.
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