The joyful mood in Britain following the coronation of King Charles III has been slightly soured by police behaviour in arresting anti-monarchist protesters not for breaking the law but for the disruption they “might” have caused.

The pre-emptive arrests of 52 protesters from Republic, an anti-monarchist campaign group, were widely condemned by human rights organisations.

Bengali baroness Shami Chakrabarti, a former director of Liberty and Labour’s shadow attorney general under Jeremy Corbyn, told BBC Radio 4 News that she had received expressions of concern from friends and contacts overseas.

Among those critical of the metropolitan police in London was the UK director of Human Rights Watch, Yasmine Ahmed, who said arresting peaceful protesters was “something you would expect to see in Moscow, not London”.

The police were using powers under new legislation brought in by the home secretary Suella Braverman.

Under the toughened Public Order Act, protesters who use handcuffs or strong glue to “lock on” to steel frames on building sites are liable to a fine, while those who block roads by lying down in front of traffic face up to 12 months in prison.

Sacha Deshmukh, the UK chief executive of Amnesty International said: “Peaceful protest is clearly protected under international human rights law and it’s been worrying to see the police making numerous statements about their ‘low tolerance’ for disruption at the coronation.”

Accusations of heavy-handed enforcement began early on Saturday before the coronation began, when the chief executive of anti-monarchist campaign group Republic, Graham Smith, was arrested in Trafalgar Square.

Footage showed protesters, including Smith, in “Not My King” t-shirts being detained around 7.30 — three and a half hours before the start of the coronation service.

The Met said “lock-on devices”, which protesters can use to secure themselves to railings, had been seized from a van. But Republic said officers had “misconstrued” straps meant to secure their signs in place.

Smith was released without charge after 16 hours in custody and tweeted that there was “no longer a right to peaceful protest in the UK. I have been told many times the monarch is there to defend our freedoms. Now our freedoms are under attack in his name”.

Positioned by London’s oldest statue of Charles I, who lost his head to Republicans nearly 400 years ago, the protesters were heavily outnumbered. But they made their presence felt with boos and chants of “Not my King” and “What a waste of money”.

The arrests were defended by Commander Karen Findlay, who led the police effort to secure the coronation: “The coronation is a once-in-a-generation event and that is a key consideration in our assessment.”