The King and Queen Consort were greeted by shouts from protesters holding a banner reading "Not my King", as they arrived for a day of celebrations marking Colchester becoming a city.
One of the protesters, speaking through a megaphone, said as Charles and Camilla got out of a car: "Answer your critics Charles, come and talk to your critics."
They called: "Why are you wasting our money?"
The King waved to the gathered crowds before going through a gate into Colchester Castle's grounds, but did not appear to acknowledge the protesters who were stood on the far side of the road behind a police presence.
The couple are visiting Colchester as part of the festivities organised to celebrate the town becoming a city as part of the Queen's Platinum Jubilee, and having its status made official in November.
They arrived at Colchester Castle late due to poor weather.
They chatted to the soldiers and dignitaries that had gathered at the gates and met some Colchester schoolchildren, who were waving Union flags.
Inside the castle Charles met local organisations which represent the city’s heritage and communities, was shown a historic vase, and an artefact trolley by youngsters.
The mayor of Colchester, Tim Young, told the King during a reception: “Words cannot describe how much it means to us that you are here.”
He said that he was pleased that the Letters Patent which officially recognise Colchester as a city have been placed in the chapel of the castle, “especially as this is the very room where the mayoress and I had our wedding just 10 days ago – in this very room, in this very suit”.
“This will always certainly be a very special place for us,” said Mr Young.
Outside the castle, Charles revealed the name of Colchester Zoo’s newest addition, baby white rhino Dara, by pulling a cord to draw back material which had covered the name on an easel.
A town crier then rang a bell and boomed, as Charles chuckled: “Oyez, oyez, oyez! I officially proclaim Colchester Zoo’s newest addition as baby Dara.”
The King told zoo staff: “I do hope it grows into a substantial animal.”
He then departed by car as a band played God Save The King.
Andy Moore, conservation education manager at Colchester Zoo, said the name Dara means "beautiful one" in Swahili.
“It was all a secret and a surprise so his majesty revealed it for us,” he said. “And then the town crier – probably people in Wivenhoe and further away could hear. It was really great.”
Charles and Camilla also visited Colchester Library, where they joined Age UK for an afternoon tea with local volunteers, service users and The Silver Line staff to hear about the support provided by the charity across the region.
Charles became patron of Age UK in in 2010 when he was Prince of Wales, while Camilla became patron of The Silver Line in 2017 when she was Duchess of Cornwall. The countywide campaign was established to help children and young people discover a lifelong love of reading, and help to ensure every Essex child is able to read at their age level or above.
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