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.
“They’re under arrest. End of,” the officer told them, walking off, according to footage posted by the group on Twitter.
On its Twitter feed, Republic confirmed the arrests and seizure of placards. “Is this democracy?” it asked.
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 police powers 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, at least 19 members of Just Stop Oil were arrested in central London, the group — which stages disruptive demonstrations — said in a statement.
An AFP reporter saw numerous activists being handcuffed by police on The Mall, the processional route from Buckingham Palace to Trafalgar Square.
“Their intention was only to display T-shirts and flags. This is a massive authoritarian overreach,” Just Stop Oil said.
“While everyone is focused on a billionaire in a shiny hat, the government is signing off on plans to destroy the lives of millions of ordinary people, while enacting laws to ensure that no one can stop them.”
The Metropolitan Police tweeted that officers had made “a number of arrests” as part of its “significant police operation” in central London.
It said four people were held at the scene of the detentions of Republic’s activists “on suspicion of conspiracy to cause public nuisance”.
Officers seized “lock-on devices” there, it added.
The arrests left other anti-monarchy demonstrators incensed.
Eva Smeeth, 19, said the new police powers law had motivated her to turn out.
“This bill is all so wrong so, yeah, I don’t feel like celebrating anything today,” she told AFP, holding a placard bearing the slogan “abolish the monarchy not the right to protest”.
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