"[Greta Thunberg] has just been arrested," the movement wrote on X, formerly known as Twitter.
A photo attached to the message shows two police officers holding the 20-year-old by the arms. The activist is seen wearing a badge that reads "Oily Money Out."
The protest took place outside the InterContinental London Park Lane Hotel, where a three-day Energy Intelligence Forum, a gathering of officials from the leading oil and gas companies, began on Tuesday.
Earlier this month, the district court in Sweden's Malmo fined Thunberg 2,250 kronor ($194) for disobeying police at a protest in the town in July. The demonstration was not authorized and its participants blocked the road. Swedish police removed the activist from the protest site after she refused to leave on her own.