Climate change activists have blocked part of the M25 motorway near Heathrow in defiance of a court order.
Insulate Britain said 52 people had blocked the slip road at junction 14 on Monday morning.
Officers were making arrests, the Metropolitan Police said.
A High Court injunction banning protests around the M25 and Dartford Crossing was issued last Thursday.
It is the sixth time the M25 has been blocked in recent protests by climate change activists.
Insulate Britain spokesperson Liam Norton said: “You can throw as many injunctions at us as you like, but we are going nowhere.
“You can raid our savings and confiscate our property. You can deny us our liberty and put us behind bars. But that is only shooting the messenger.
“The truth is that this country is going to hell unless you take emergency action to stop putting carbon into the air.”