A group of anti-monarchy campaigners on Saturday staged what organizers are calling the "first-ever" 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," Britain's largest anti-monarchist group, Republic, said in a statement.

The group released a photo where protesters can be seen standing inside the palace wearing T-shirts that spelled out "Not My King."

"The protest is the latest in a series of actions aimed at pushing forward the debate about the future of the monarchy," Republic said in a statement.

The group said six of the activists involved were briefly detained by security, before being escorted out of the front gate.

Buckingham Palace told CNN that it doesn't comment on security matter. CNN has reached out to the London's Metropolitan Police for comment.

The organizers described the protest as a "fantastic statement of intent, citizens standing up in the home of the monarchy to declare their opposition to hereditary power."

The group's chief executive Graham Smith in a statement called King Charles III "an untouchable monarch" who is not "immune to criticism and doesn't enjoy the deference that protected the monarchy while his mother was on the throne."

"Republic will continue to protest against the monarchy up and down the country, with the next protest set for the state opening of parliament on November 7," Smith added.

The group was also involved in organizing anti-monarchy protests around King Charles' coronation ceremony earlier this year.