LONDON, May 6 (AFP) - UK police on Saturday arrested leading members of the anti-monarchy group Republic as they prepared to protest along the route of a procession for the coronation of King Charles III, the organisation said.

"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 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 UK 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, according to footage posted by the group on Twitter.

UK police forces were controversially granted new anti-protest powers by the 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.

Smith told reporters last week: "We certainly have no plans to disrupt the actual procession."

Waving placards and shouting would show "in front of the world's press that we are not a country of loyalists, that there is a growing opposition", the Republic founder said.

The new law was enacted after months of disruptive tactics around Britain by groups opposed to fossil fuels. It entails stiffer jail terms against activists gluing and padlocking themselves to immovable objects.

Separately Saturday, 13 members of the group Just Stop Oil were arrested and handcuffed by police on The Mall between Trafalgar Square and Buckingham Palace, an AFP reporter saw.

In a statement, the Metropolitan police confirmed only seven arrests early Saturday "on suspicion of offences including breaching the peace and conspiracy to cause public nuisance close to the coronation".

Officers seized "lock-on devices" from one group next to Trafalgar Square, it said.