Police who arrested anti-monarchy protesters before King Charles III’s coronation have “destroyed whatever trust might have existed between peaceful protesters and the Metropolitan police,” the chief executive of the campaign group Republic has said. After six members of the group were arrested at about 7.30am on Saturday – before their protest had begun – and had their placards seized, Graham Smith said officers “should hang their heads in shame” and that police had shown “no judgment, no common sense and no basic decency”. Smith and his fellow campaigners for an elected head of state were held at a police station until late on Saturday night despite previously liaising with Scotland Yard. After his release, Smith said: “What is the point in being open and candid with the police, working with their liaison officers and meeting senior commanders, if all their promises and undertakings turn out to be a lie? “It is notable that King Charles has said nothing about these arrests. Rather than defend our liberty and values, he is content celebrating his anointment as monarch while citizens are locked up.” Asked what further action he and other members of Republic faced, Smith said: “We don’t know. We’ve all been bailed, that’s all we know. So, we will hear about whatever happens later.” Scotland Yard said officers made 52 arrests on Saturday for offences including affray, public order offences, breach of the peace and conspiracy to cause a public nuisance. “Lock-on” devices were seized during the arrest of the Republic organisers, a police statement said, but the group denied having any such equipment. It claimed one protester was arrested for possession of string that was part of a placard. Sadiq Khan, the mayor of London who is responsible for policing in the capital, said some of the arrests “raise questions and whilst investigations are ongoing, I’ve sought urgent clarity from Met leaders on the action taken”. Adam Hug, the Labour leader of Westminster city council, also said he was “urgently pushing the police for proper answers” after three council volunteers were arrested at about 2am on Saturday in Soho and later released on bail after they were found in possession of rape alarms. Police said “military colleagues” had believed such devices could be used to disrupt parading horses, posing “significant risk to the safety of the public and the riders”. The council, however, said the devices were for the Night Stars volunteers to give out to vulnerable women and they were funded by a Home Office grant. They also gave out flip-flops, vomit bags and water to revellers in need. “It seems very likely the Night Stars have got caught up in a blanket presumption of arresting anyone with potentially disruptive devices even if they had them for lawful or approved purposes, an approach the Met had not briefed partners about,” a senior council source said. “Because the arresting officers weren’t local, they didn’t know who the Night Stars were and didn’t know to call the council to try and de-escalate.” Earlier, a minister defended the arrests, claiming police were right to act as “we were on the global stage”. Lucy Frazer, the culture secretary, told the BBC’s Sunday with Laura Kuenssberg. “I think they were quite right to take into account the context of the event as a whole. “There were 200 foreign dignitaries in the UK, in London, at an event with millions of people watching, and hundreds of thousands of people at the scene. I think it was really important that they took that into account when making their decision.” Frazer said the Met was operationally independent and had to make “tough calls”, and she claimed new protest laws introduced last week were a necessary response to “a change in tactics of protesters”. She said: “We have seen them stopping people going about their day-to-day business, whether that’s going to school, being able to go to hospital, being able to go to work.” The former justice secretary Robert Buckland told GB News: “The police would be in the dock if they let something happen and the parade was obstructed or horses were frightened. We can’t, that’s not safe.” But Sue Sim, a former chief constable of Northumbria police, told BBC Radio 4 she was disappointed by the arrests and said: “We do not want a totalitarian police state.” Scotland Yard said its tactics were proportionate. Commander Karen Findlay, who led the policing operation, said: “We absolutely understand public concern following the arrests we made. Protest is lawful and it can be disruptive. We have policed numerous protests without intervention in the build-up to the coronation, and during it. Our duty is to do so in a proportionate manner in line with relevant legislation. We also have a duty to intervene when protest becomes criminal and may cause serious disruption. This depends on the context. The coronation is a once-in-a-generation event and that is a key consideration in our assessment. “A protest involving large numbers has gone ahead … with police knowledge and no intervention.” Smith, who has campaigned against the monarchy for two decades, claimed the arrests showed the right to protest peacefully in the UK no longer existed, but he said Republic would keep protesting “wherever Charles goes, wherever William goes … with one simple message: Charles is not our king, it is time to abolish the monarchy”. The arrests triggered accusations from campaigners and human rights groups that the freedom to protest was under attack. Amnesty International UK’s chief executive, Sacha Deshmukh, said: “Being in possession of a megaphone or carrying placards should never be grounds for a police arrest.” Human Rights Watch said reports of arrests of peaceful protesters were incredibly alarming and “something you would expect to see in Moscow, not London”. On Sunday, Ed Davey, the leader of the Liberal Democrats, said: “I don’t know the exact reason the police arrested those people … but I hold the Conservative government responsible for passing legislation to clamp down on protests that breached British traditions of civil liberties and I think the Conservatives have got a lot to answer for.” Jason Arday, a professor of sociology and education at Cambridge University, said of arrests of protesters in Trafalgar Square: “What we are beginning to see is an infringement of our ability to have free speech. I think it was a peaceful protest and to be quite honest, and without surprise, I think the police’s reaction to that particular situation was pretty heavy-handed and rather unnecessary.”