The man, holding a blank sheet of paper, as seen in the footage below, shouted, “No more monarchy” at the couple as they greeted locals in the crowd.
The lone protestor, who did not want to give his name, was held back by police who he said “were worried that people were going to get annoyed and attack”.
Speaking afterward, he added: “It is the 21st century, we don’t need a monarchy. It’s ridiculous.”
Prince William and Princess Kate seemed unfazed by the minor disruption as they spoke with members of the public.
After leaving the National Maritime Museum Cornwall where they tried their hand at boat restoration, Kate was surprised by Jim Embury, a former teacher from her prep school.
Kate said, “oh my goodness” and hugged Mr Embury, now a volunteer at the museum, before exclaiming: “I do recognize you.”
“I remember the classroom and everything,” she told him. “Are you based here now? And you are volunteering here? Wow. That is such a small world.
“I’m trying to teach my daughter all the things you probably taught me.”
Speaking after the meeting, Mr Embury said he taught Kate history in the mid-1990s.
When asked what sort of pupil she was, he replied: “I have to say fantastic. It was a great class and she was a great participant and a great kid. It was 25 years ago.”