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.
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Local newspaper 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.”
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“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.
The monarch, who would officially be crowned alongside his wife Queen Consort Camilla on May 6 in London, spent few minutes with the Union Jack-waving crowd.
The King then walked inside the church with a huge smile on his face to attend the reception attended by members of the local community and Milton Keynes officials.
Charles’ visit to Milton Keynes was announced earlier this month. Camilla was also supposed to accompany her husband, but she had to cancel her plans earlier this week after testing positive for COVID-19.