Protesters appear to have been arrested ahead of the King's coronation.
Footage on Twitter seemed to show Graham Smith, chief executive of the anti-monarchy group Republic, being apprehended by police in St Martin's Lane, near Trafalgar Square, on Saturday morning.
Pictures appeared to show demonstrators in yellow "Not My King" T-shirts, including Mr Smith, having their details taken by officers.
In one video, an officer says: "I'm not going to get into a conversation about that - they are under arrest, end of."
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On Wednesday, the Metropolitan Police announced they would have an "extremely low threshold" for protests during the coronation celebrations, and that demonstrators could expect "swift action".
After the apparent arrests, Republic activist Luke Whiting, 26, told the PA news agency: "Six Republic members have been arrested including the CEO as the demonstration was starting at the edge of Trafalgar Square.
"It is unclear why, potentially it is because one of them was carrying a megaphone. It is unclear exactly whether the police are using these new powers and whether they are misusing them to stop protest happening."
One woman in a Republic T-shirt who was arrested told PA: "We had a delivery of placards ready for the protest and then the tactical support unit questioned us as to how we had got through the road closures."
Officers carried her away from where she had been standing outside a Tesco store. Two men who appeared to have been part of the same demonstration were carried away by officers into a marked police van.
The policing operation is set to see 11,500 police officers on duty on Saturday.
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