Just Stop Oil activists have been accused of blocking an ambulance with "blue lights on" as it tried to weave its way through a demonstration on Waterloo London.
Police made at least 40 arrests after activists refused to move away with some slow marching towards The Strand.
The Metropolitan Police said the march on Wednesday was causing traffic to be held up on the bridge, including an ambulance with "blue lights on".
But Just Stop Oil posted on X, formerly Twitter, claiming that it was police officers who appeared to be blocking the ambulance "so they can blame it on a protest march going in the other direction".
Police posted a picture of the disruption and outlined the situation they were facing.
They said: "This is some of the congestion which JSO are causing on Waterloo Bridge.
"One of the vehicles is an ambulance on blue lights which is not able to get past.
"Officers are continually telling the activists to move out the road so it can pass while making arrests."
Police had earlier warned activists slow marching on Waterloo Bridge that if they did not move out of the road to continue their demonstration, they would make arrests.
A group of five Just Stop Oil activists, who were part of the original demonstration on Waterloo Bridge, then moved the slow march to The Strand, officers said.
"These are the final activists in the road. Officers are with them and making arrests," they added.
It comes after 44 Just Stop Oil activists were charged following disruption in London on Monday.
The charges include two people who allegedly smashed the glass cover of the Rokeby Venus, a painting once slashed by a suffragette at the National Gallery.
The other 42 activists were charged with obstruction of the highway
On Monday, police made around 100 arrests in Whitechapel as activists took part in a slow march, some of which was held around the Cenotaph.
The campaign group was accused of targeting the war memorial - which it strongly denied, saying activists had been moved to its base by police officers after shutting down traffic on the road.
JSO is calling on the government to stop all new gas and oil projects in the UK in order to tackle the climate crisis.
It is currently carrying out a four-week campaign of demonstrations.
On 1 November, 35 people were arrested after marching down West Cromwell Road in Kensington, and a further 60 were arrested in Parliament Square two days before, on 30 October.
