Scotland Yard has been accused of an “incredibly alarming” attack on the right to protest after police used new powers to arrest the head of the leading republican movement and other organisers of an approved demonstration just hours before King Charles III’s coronation. Graham Smith, the chief executive of Republic, had been collecting drinks and placards for demonstrators at the main site of the protest on Trafalgar Square two hours before the king was due to arrive at Westminster Abbey when he was stopped along with five others by police on nearby St Martin’s Lane. The group had been walking behind a rental van containing hundreds of placards when they were approached by the police and searched. A video of an exchange with one of those arrested was caught on film. One police officer can be heard saying: “I’m not going to get into a conversation about that, they are under arrest, end of.” The arrests were said to have been made despite a series of meetings and agreements between Republic and Scotland Yard over the demonstration, which was to take place at the point where Whitehall meets the Mall. The Metropolitan police later said a total of 52 arrests had been made for affray, public order offences, breach of the peace and conspiracy to cause a public nuisance around the coronation. Three people were arrested in the early hours of the morning in Soho, the Met police said, after intelligence indicated groups and individuals were planning to use and throw rape alarms in order to disrupt the coronation procession. There was concern that this would scare horses involved in the procession and, as a result, risk the safety of the public and the riders, the Met said. But Westminster city council said it was “deeply concerned” by reports that volunteers who work on its women’s safety campaign Night Stars were arrested. The three people – a 37-year-old woman, a 59-year-old woman and a 47-year-old man – were taken to a south London police station, where they were questioned, the police said. Among items seized during the arrest were a number of rape alarms. The 47-year-old man was also further arrested on suspicion of handling stolen goods. All three have since been released on bail pending further inquiries. Councillor Aicha Less, cabinet member for communities and public protection at the council, said: “We are deeply concerned by reports of our Night Stars volunteers being arrested overnight. This service has been a familiar and welcome sight in the West End for a long time and [its workers] have extensive training so they can assist the most vulnerable on the streets late at night. “We are working with the Metropolitan police to establish exactly what happened, and in the meantime, we are in touch with our volunteers to ensure they are receiving the support they need.” Cmdr Karen Findlay, who was leading “Operation Golden Orb”, the police effort to secure the coronation, said the force had acted on the understanding that it was a “once in a generation” event. She said: “All of these people remain in custody. We absolutely understand public concern following the arrests we made this morning. Protest is lawful and it can be disruptive. We have policed numerous protests without intervention in the buildup 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.” Harry Stratton, a director at Republic, who arrived as Smith and the other protest organisers were detained, said: “They were collecting the placards and bringing them over when the police stopped them. The guys asked why and they were told: ‘We will tell you that once we have searched the vehicle.’ That’s when they arrested the six organisers. “We asked on what grounds they had been arrested but they wouldn’t say. It is a surprise as we had had a number of meetings with the police. They had been making all the right noises.” Human rights activist Peter Tatchell said the police had reneged on private assurances that the anti-monarchist protest could go ahead unimpeded. He said: “They have gone back on these promises by arresting the head of Republic, seizing their placards and megaphones, submitting those here to photographic surveillance and constructing a watchtower in front of the demonstration so that the king would not see the protest as he passed by on the way to the palace.” A wall was also constructed around Trafalgar Square mid-morning that blocked off many late-arriving protesters from joining the demonstration. They instead held a march around the perimeter. Scotland Yard later said “several” arrests had been made for breach of the peace and conspiracy to cause public disorder, adding that lock-on devices used by protesters to attach themselves to street furniture had been found. The allegation was denied by Republic. A Met spokesperson said: “We have made a number of arrests in the area of Carlton House Terrace. The individuals have been held on suspicion of breaching the peace. Earlier today we arrested four people in the area of St Martin’s Lane. They were held on suspicion of conspiracy to cause public nuisance. We seized lock-on devices. “A further three people were arrested in the area of Wellington Arch. They were held on suspicion of possessing articles to cause criminal damage. There will be further updates.” Stratton said the organisers of the protest had not possessed lock-on devices. “What would we lock on to? We are just protesting.” He added that one protester at Trafalgar Square had been taken away by police as he had string on him. “It’s string that was part of his placard, he said. “What was he going to do with that?” The Met police had tweeted earlier this week that they would have an “extremely low tolerance” of those seeking to “undermine” the day. Under the new Public Order Act, protesters who have an object with the intention of using it to “lock on” are liable to a fine, with those who block roads facing up to 12 months in prison. Yasmine Ahmed, the UK director of Human Rights Watch, condemned the arrests. “The reports of people being arrested for peacefully protesting [against] the coronation are incredibly alarming. This is something you would expect to see in Moscow, not London. Peaceful protests allow individuals to hold those in power to account, something the UK government seems increasingly averse to,” she said. Sacha Deshmukh, Amnesty International UK’s chief executive, said the human rights group had been concerned about Met statements about its “low tolerance” of protests. He 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. “Peaceful protest is clearly protected under international human rights law and it’s been worrying to see the police this week making numerous statements about their ‘low tolerance’ for disruption at the coronation. The coronation shouldn’t become yet another excuse for undermining people’s basic human rights in this country and we’re awaiting more details over these concerning reports of arrests.” Just Stop Oil said that about 13 of their protesters had been arrested on the Mall. A spokesperson for the campaign group said five demonstrators were also arrested at Downing Street and one on Piccadilly. In one exchange caught on camera, the Just Stop Oil campaigner Ben Larsen, 25, told officers: “You’ve searched me and haven’t found shit.” A police officer responded: “You need to educate yourself on what peaceful protest is.” Separately, Animal Rising said a number of their supporters were arrested on Saturday morning while at a training session “miles away from the coronation”. Nathan McGovern, a spokesperson for the campaign group, said: “This is nothing short of a totalitarian crackdown on free speech and all forms of dissent. “Just Stop Oil, Republic and Animal Rising have experienced the true character of this government’s attitude towards peaceful protest today. We are sleepwalking into fascism and it is every single person’s responsibility to stand up and say ‘No more.’” Smith’s arrest, at about 7.30am, had come as hundreds of anti-monarchist protesters were gathering at Trafalgar Square with large flags and wearing yellow T-shirts as they tried to catch the eye of the world’s media and that of a king on his coronation day. 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 the 2,000 demonstrators lining that part of the procession route made their presence felt with boos and chants of “Not my King”. The protesters heckled throughout the procession and chanted “What a waste of money”, but they did so peacefully. The crowd of protesters was joined under Nelson’s Column by representatives of the Swedish, Dutch and Norwegian republican movements. Maria Gomez, 39, from Borehamwood, Hertfordshire, said there had been some intimidation aimed at the protesters. “Some were shouting ‘Burn the yellow flags’ earlier but we have had intimidation before – there were eggs thrown in 2012,” she said. “People can get very angry with republicans.” The protest drew many well-known characters of the anti-monarchy movement, including Patrick Thelwell, 23, who was found guilty last month of a public order offence for throwing five eggs at the king during a visit to York. As King Charles was being anointed and crowned in the abbey, Thelwell was seized from the crowd by police officers, handcuffed and searched on “suspicion of having prohibited items”, he said. “They saw me on their watchtower and next minute I was in handcuffs and being searched on suspicion of having eggs, I suppose,” he said. “I didn’t have anything in my pockets apart from condoms and a lighter so they had to let me go.” “I’ve absolutely not brought any eggs,” he added. “My parole officer tells me counter-terrorism [policing] is following me.” Paul Powlesland, the lawyer who was threatened with arrest when holding up a blank piece of paper after the death of the queen, was also in the crowd. Powlesland said that the arrest of Smith had been an attack on the right to protest. “What could be more British than making speeches in the street?” he said.