The United Kingdom police on Saturday arrested 52 people, including Graham Smith, leader of an anti-monarchist group Republic as they prepared to protest along the route of a procession for the coronation of King Charles III.

The protesters were detained for a range of offenses including affray, breach of the peace and conspiracy to cause a public nuisance.

In a statement, the London police force said all those who had been held remained in custody.

"They've arrested six of our organisers and seized hundreds of placards, they won't tell us why they've arrested them or where they're being held," a Republic activist told Agence France-Presse (AFP) in London's Trafalgar Square.

Republic chief executive Graham Smith was one of those arrested before the group had a chance to wave placards declaring: "Not My King."

Some onlookers nearby shouted "free Graham Smith!" But others shouted "God save the king" and waved Union flags.

A camera crew from the group Alliance of European Republican Movements was at the scene and asked a senior police officer why the group had been detained.

"They're under arrest. End of," the officer told them, walking off.

There was no immediate comment from London's Metropolitan police force, which was controversially granted new anti-protest powers by the U.K. government under a new law rushed through this week.

On its Twitter feed, Republic confirmed the arrests and seizure of placards. "Is this democracy?" it demanded.