London CNN  — 

London’s Metropolitan Police said it made 52 arrests during the coronation of King Charles III on Saturday, as the force faces growing scrutiny over its attitude toward anti-monarchy demonstrators.

Thousands gathered in central London on Saturday to celebrate the once-in-a-generation occasion. But it also drew demonstrators, with protesters wearing yellow T-shirts booing and shouting “Not My King” throughout the morning.

Republic, Britain’s largest anti-monarchy group, told CNN that police – without providing any reason – arrested organizers of the anti-monarchy protest.

At around 7 a.m. (2 a.m. ET) police stopped six of Republic’s organizers and told them they were detaining and searching them, Republic director Harry Stratton told CNN at the protest.

Graham Smith, the chief executive of Republic, was among those detained, according to a video shared by the Alliance of European Republican Movements.

Protesters hold signs reading "Not My King" and "Abolish the Monarchy" close to the procession.

Stratton said that when the organizers asked police why they were being detained, they were told officers “would figure it out” after they had searched the anti-monarchy protesters. After searching them, police told the six organizers they were arresting them and seizing hundreds of their placards carrying the slogan “Not My King.”

“They didn’t say why they were arresting them. They didn’t tell them or us where they were taking them. It really is like something out of a police state,” Stratton said.

“I think people are quite perturbed by the police reaction. But the crowd reaction to us has been overwhelmingly friendly,” he added.

The group posted on Twitter Saturday, commenting: “So much for the right to peaceful protest.”

Members of environmental activist group Just Stop Oil also appeared to have been arrested on The Mall outside Buckingham Palace, the UK’s PA Media news agency reported, adding that a large group of the protesters were seen in handcuffs.

A Just Stop Oil member was arrested and carried away by police.

The Metropolitan Police confirmed several arrests had been made in central London and defended its actions.

“A total of 52 arrests have been made today for offenses including affray, public order offenses, breach of the peace and conspiracy to cause a public nuisance. All of these people remain in custody,” the police said in a press release.

Met defends arrests

Commander Karen Findlay, who is leading the police operation, said in the release: “We absolutely understand public concern following the arrests we made this morning.

“Protest is lawful and it can be disruptive. We have policed numerous protests without intervention in the build-up to the coronation, and during it.

“Our duty is to do so in a proportionate manner in line with relevant legislation. We also have a duty to intervene when protest becomes criminal and may cause serious disruption.

“This depends on the context. The coronation is a once in a generation event and that is a key consideration in our assessment. A protest involving large numbers has gone ahead today with police knowledge and no intervention.”

Human Rights Watch, a non-profit campaign group, said earlier Saturday that the coronation arrests were “something you would expect to see in Moscow not London,” according to a statement obtained by PA Media.