Six people, including two Olympic athletes, were detained on Saturday for scaling and gluing themselves to an oil tanker near Hyde Park.
Protesters also scaled Marble Arch and hoisted a green banner reading “end fossil fuels now”, while other demonstrators blocked the road.
By 21:00 BST 40 people had been held, the Met said.
Olympic gold medal-winning canoeist Etienne Stott and sailor Laura Baldwin were among those who glued themselves to the oil tanker.
The action came as activists gathered in Hyde Park and then marched the capital’s streets.
Protesters waved flags, set off flares and danced in the street before settling in the road at Marble Arch.
A group of eight activists later locked themselves to a car in the road, while two glued themselves to the roof.
The Met said officers had imposed a public order direction on the group to clear the road by 18:00.
It covered an area from Marble Arch gyratory and along Park Lane, Oxford Street, Tyburn Way, Edgware Road, Bayswater Road and Cumberland Gate.