London commuters physically dragged Extinction Rebellion protesters off the top of a busy underground train on Thursday morning.
A video shared widely on social media appeared to show a number of protesters on top of a Jubilee Line train at Canning Town station in East London.
A busy platform of commuters were seen jeering at the activists before one protester is pulled down from the train.
Climate activists had planned to target London's underground network on Thursday, as part of their ongoing demonstration in the capital city.
Extinction Rebellion said the actions were "intended to bring economic disruption to the capital as part of the ongoing campaign to convince the Government to take meaningful action on the Climate and Ecological Emergency."
At around 8:00 a.m., British Transport Police said via Twitter that it had made four arrests after obstruction incidents at Canning Town and Stratford.
The police said it would remain at Shadwell station, on the Docklands Light Railway (DLR) metro system, where specialist teams were working to remove four other protesters.
In a statement published before the incident, the British Transport Police said senior officers had engaged with Extinction Rebellion to try to persuade them into stopping "highly disruptive and potentially criminal action."
"The Tube and rail networks are one of the greenest transport methods in London, any action goes against what they campaign for and will only cause misery for London's commuters," Sean O'Callaghan, assistant chief constable of the British Transport Police, said in a statement on Wednesday.
"In addition to this, we've made it clear to Extinction Rebellion that disrupting Tube services could create a dangerous scenario where some trains are stuck within tunnels with hundreds of passengers on board."
"Likewise, trespassing or obstructing Tube services could create a serious hazard to protestors," O'Callaghan said.
Earlier this week, the Metropolitan Police had issued a city-wide ban on Extinction Rebellion's climate change protests, warning activists must immediately halt their demonstrations or face arrest.
That decision came after more than a week of civil disobedience in London, with activists targeting government buildings and major financial institutions.
The protests, which began last Monday and were due to last two weeks, have resulted in more than 1,600 arrests.
In a statement on its website, Extinction Rebellion said Thursday that an "incident of violence" had taken place against a protester in Canning Town, but participants "understood that this was a possibility."
A spokesperson for the group told CNBC via email that it did not believe anyone had been injured as a result of the protests.
London Mayor Sadiq Khan said he "strongly" condemned the Extinction Rebellion protesters who have targeted the London Underground on Thursday morning.
"This illegal action is extremely dangerous, counterproductive and is causing unacceptable disruption to Londoners who use public transport to get to work."
"It is also an unfair burden on our already overstretched police officers. I urge demonstrators to protest peacefully and within the boundaries of the law."
Extinction Rebellion says it has held the so-called "Autumn Uprising" across major cities across the globe to rebel against the world's governments for their "criminal inaction" when it comes to the intensifying climate crisis.