The 20-year-old activist, who has become a key face of the movement to fight climate change, was taken away by two police officers and put into the back of a police van outside the Energy Intelligence Forum.
"Behind these closed doors, spineless politicians are making deals and compromises with lobbyists from destructive fossil fuel industry," Thunberg told journalists outside the venue hosting the annual gathering, which runs until Thursday. "I've got six grandchildren. I have nightmares about the future for them," protestor Doro Marden told AFP.
"Oil companies have racked up billions upon billions of profit, breaking records across the board last year. Oily CEOs took home multi-million pound pay cheques," Fossil Free London said in a statement.