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.

Republicans demonstrate at Trafalgar Square during the coronation of King Charles III
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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.

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.