WATCH: Police arrest protesters ahead of coronation procession in central London

London police have been accused of stifling free speech after they arrested more than a dozen environmental activists and six anti-monarchy protesters ahead of the coronation procession in central London on Saturday.

Police detained at least 19 environmentalists as they shouted messages about climate change. Some were wearing T-shirts reading 'Just stop oil'.

Anti-monarchy group Republic said several of its members were stopped near Trafalgar Square and led to police vans.

Hundreds of the yellow "Not my King" signs were also seized, the organisation said with photos.

"This is something you would expect to see in Moscow, not in London," Human Rights Watch said.

The police confirmed that they had made several arrests on suspicion of disturbing the peace in several places.