UK police drew condemnation Saturday after arresting leading members of the anti-monarchy group Republic as they prepared to protest along the procession route for the coronation of King Charles III.

London's Metropolitan Police force detained six organisers from the pressure group and seized hundreds of their placards, Republic said.

Republic chief executive Graham Smith was among those detained near Trafalgar Square, before the group had a chance to wave the signs declaring "Not my king".

"The whole core team of Republic is still being detained," the group said on Twitter some seven hours after the arrests and well after the coronation ceremony. "Is this democracy?"

The Met tweeted that four people were held "on suspicion of conspiracy to cause public nuisance".

"We seized lock-on devices," it added, referring to newly outlawed contraptions used by demonstrators to attach themselves to each other, an object or the ground.

But the detentions prompted swift 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."

'Dystopian'

The arrests came just days after UK police forces were controversially granted new anti-protest powers by the government following years of disruptive demonstrations by environmental activists.

It expands protest-related offences to include locking-on and carrying lock-on devices, extends police stop and search powers, and allows for new court orders to prevent people from attending demonstrations.

Separately on Saturday, at least 19 members of Just Stop Oil were also arrested in central London, the environmental campaign group said in a statement.

An AFP reporter saw several activists being handcuffed by police on The Mall, the processional route from Buckingham Palace.

"Their intention was only to display T-shirts and flags. This is a massive authoritarian overreach," Just Stop Oil said, claiming that none of those arrested had "glue, paint or any plans to disrupt the coronation".

"New policing laws mean we're now living in a dystopian nightmare -- this disgraceful overreach is what you'd expect in Pyongyang, North Korea, not Westminster."

Reports said other protesters were detained, including a man allegedly held in St James's Park for carrying a megaphone.

"It could scare the horses," a police officer at the scene said, according to a Sky News television reporter present.

Amnesty International's chief executive Sacha Deshmukh said: "We need to see what details emerge around these incidents, but merely being in possession of a megaphone or carrying placards should never be grounds for a police arrest."

'Wrong'

Republic, which wants Britain's constitutional monarchy replaced by an elected head of state, had been vocal about its protest plans, but Smith said this week that they had no plans to disrupt the procession.

Republic's director Harry Stratton, 30, said activists were carrying placards near Trafalgar Square when around 20 officers stopped and searched them.

"Graham and our volunteers asked why and they said 'we will find that out'," Stratton said. "After that they arrested them, saying 'we are seizing all these placards'."

A camera crew from the group Alliance of European Republican Movements asked a senior police officer why they were being held.

"They're under arrest. End of," the officer replied, according to footage posted on Twitter.

The Met, which this week had vowed "low tolerance for any disruption", tweeted that officers made "a number of arrests" as part of its "significant police operation".

Dubbed "Golden Orb", it deployed 11,500 officers on Saturday as well as facial-recognition technology that civil liberties organisations branded "authoritarian".

The detentions infuriated the scores of other demonstrators who rallied near The Mall as well as in Trafalgar Square.

"This bill is all so wrong so, yeah, I don't feel like celebrating anything today," Eva Smeeth, 19, told AFP, holding a placard bearing the slogan "abolish the monarchy not the right to protest".