Loondon, May 6 : The head of anti-monarchist campaign group Republic has been arrested at a protest in Trafalgar Square ahead of the Coronation of King Charles III. Footage shows protesters in "Not My King" t-shirts being detained, including Republic's CEO Graham Smith, the BBC reported. Six demonstrators, including Smith, were arrested by police while unloading placards near the Coronation procession route, the group said. Republic posted photos of officers taking details from them on Twitter. "So much for the right to peaceful protest," the group said, adding the officers would not give the reasons for their arrest and confirmed their CEO was among them. Matt Turnbull, one of those arrested, said the straps holding the placards had been "misconstrued" as something that could be used for locking on, the BBC reported. "To be honest we were never going to be allowed to be a visible force here -- they knew we were coming, and they were going to find a way to stop this," he told the BBC. The BBC later saw Turnbull being led away in handcuffs. New legislation passed this week made it illegal to prepare to lock-on to things like street furniture. The Metropolitan Police has not confirmed the exact number of people detained, but did say a number of arrests had been made near the Coronation parade route. Several people have been detained on suspicion of breaching the peace on Carlton House Terrace, near St James' Park, the BBC reported. Four persons have been held on suspicion of conspiracy to cause public nuisance in the area of St Martin's Lane, by Trafalgar Square, with lock-on devices seized while three more were held on suspicion of possessing articles to cause criminal damage near Wellington Arch. On Friday, Smith said he had been speaking with the Met to organise the protest since January and he was not concerned he would be arrested, as the new laws would not apply to their plans. He told BBC Radio Leeds that Republic "certainly have no intention of actually disrupting" the proceedings. "We will be very visible, we will be loud, we will be hard to miss, but the procession and the plans for the coronation will go ahead uninterrupted by us." Republic said on Saturday morning that hundreds of their placards had been seized and questioned: "Is this democracy?" "Some ask why we're protesting. It's because we want to use the coronation to change the debate about the monarchy and show that we're not a nation of royalists," the campaign group wrote. The anti-monarchy protest was arranged near Trafalgar Square in central London, with the crowds bursting into chants of "Not my King" and "Free Graham Smith". Thousands of people have gathered to watch the Coronation procession, Buckingham Palace to Westminster Abbey ahead of the ceremony. Around 13 'Just Stop Oil' protesters appear to have been been arrested on the Mall, with a large group from the climate change campaign group seen in handcuffs. 'Just Stop Oil' has said five demonstrators were also arrested at Downing Street. A spokeswoman for the group said their plan was "only to display T-shirts and flags", adding: "This is a dystopian nightmare." Non-profit campaign group 'Human Rights Watch' said the arrests were "something you would expect to see in Moscow not London". "The reports of people being arrested for peacefully protesting the coronation are incredibly alarming", its UK director Yasmine Ahmed said in a statement. /IANS