The King and Queen Consort were greeted with chants of “not my king” at York Minster as they arrived for the monarch’s first Royal Maundy service.
The King was visiting the cathedral on Thursday to distribute commemorative coins to 148 people who have made a difference to their communities.
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Among cheers from the crowds that lined the street to greet the royal couple, a group of 30 demonstrators from the group Republic, held up bright yellow placards and shouted “not my king” as they arrived.
The Republican protesters have also said they will make themselves “unmissable” during the day of the King’s coronation.
Demonstrators received a “mixed reaction” from the crowds, the majority of whom did not agree with their message that the head of state should be elected, said Graham Smith, the chief executive of the campaign group.
As the service took place, the crowd around the cathedral grew to thousands and, after the royal couple emerged, they spent 20 minutes talking to the crowd in a lengthy walkabout.
Mr Smith continued to say that the group will create an even bigger and “unmissable” scene along the procession route, where the King and Camilla will travel from Buckingham Palace to Westminster Abbey.
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“We’ll be protesting at the coronation and beyond just to really challenge the idea of a succession of head of state and to say we want an election instead of a coronation – a choice instead of chance,” he said.
“We will be making some noise. There’ll be a bit of a party atmosphere, trying to engage the people around us and keep it fairly light-hearted.
“And we’ll make sure that when the procession goes past we are unmissable.”
Mr Smith said he expects more than a thousand people from the group to attend on Saturday 6 May.