The actor was pictured speaking to demonstrators who were preparing to spend the night outside in Trafalgar Square on Wednesday night.
Activists said the Sherlock star spent a “good couple of hours” with the protesters, who were sitting in front of a hearse containing a coffin that carried the words: “our future”.
Cumberbatch is currently in the capital filming for Louis Wain, which he is starring in alongside Claire Foy.
XR are protesting in cities around the world, including Berlin, Paris, Amsterdam and Sydney, and are calling for urgent action on global climate and wildlife emergencies.
The group are today set to begin a three-day "Hong Kong-style" occupation of London City Airport, with organisers saying its members intend to "peacefully occupy and shut down" the terminal building from 9am.
Activists said they will achieve this by lying, sitting or gluing themselves in front of the departure and arrival gates at the east London airport.
The latest protest is against the climate impact of flying and the Government's ongoing support of airport expansion.
If they do not make it into the building, they plan to occupy the airport's access road and DLR station.
Extinction Rebellion spokesperson Rupert Read said: "By non-violently shutting down this airport, in homage to the style of the Hong Kong democracy protesters, we are demonstrating the utter frailty of the transport systems that countries such as ours, unwisely, have come to depend upon."
The group said hundreds of people have already signed up to take part in the protest, which will seem them "non-violently use their bodies" to close the airport.
It said those taking part are willing to risk being arrested to achieve their aim.
Hundreds of police officers are being drafted in from across England and Wales to help the Metropolitan Police with the protests, which are now in their third day.
Activists have been camped on rounds around Parliament Square and Whitehall since Monday calling for urgent action on climate change and wildlife.
The Metropolitan Police confirmed that 800 people had been arrested since Monday, with another 220 detained on Wednesday.
Scotland Yard deputy assistant commissioner Laurence Taylor said there are plans in place to intervene and "deal proactively" with anyone who has the intention of shutting down the airport.
A London City Airport spokesman said they were working with the police to prepare for the protests and all passengers would be required to show their boarding pass to access the terminal.