UK police drew condemnation after arresting 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 on Saturday.

Officers from London's Metropolitan Police force detained six organisers from the pressure group and seized hundreds of their placards, Republic said, just hours before Charles's crowning.

Republic chief executive Graham Smith was one of those held 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.

"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.

The detentions prompted immediate criticism from Human Rights Watch, which called the arrests "incredibly alarming".

"This is something you would expect to see in Moscow, not London," the rights organisation's UK Director, Yasmine Ahmed, said in a statement.

"Peaceful protests allow individuals to hold those in power to account -- something the UK government seems increasingly averse to."

The arrests come just days after UK police forces were controversially granted new anti-protest powers by the government under a new law rushed through this week.

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.

(AFP)

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