"Just after midday today, members of Republic staged a protest inside Buckingham Palace. The group of local activists from different parts of the UK visited the palace as tourists, before standing in the Grand Hall wearing t-shirts that spelled out ‘Not My King," the movement said in a statement.
As a result, security of the Palace briefly detained six protesters and escorted them out through the main gate, the statement added.
"This is a first, an anti-monarchy protest inside Buckingham Palace. A fantastic statement of intent, citizens standing up in the home of the monarchy to declare their opposition to hereditary power," the head of Republic, Graham Smith, said.
"Charles is not an untouchable monarch, he is not immune to criticism and doesn't enjoy the deference that protected the monarchy while his mother was on the throne," Smith added.
He noted that the movement would continue to protest against the monarchy throughout the country, with the next protest scheduled for November 7.