Ten protesters have blocked three roads leading to one of the biggest oil refineries in Europe.
They arrived and set up their blockade in the early hours of Thursday morning at the Valero site in Milford Haven, Pembrokeshire.
Dyfed Powys Police said they were first alerted at around 5.45am.
The protesters are lying down on the road at all three entrance sites.
It comes two months after the same group of climate protesters blocked one of Cardiff city centre's main roads.
Castle Street was closed for three days resulting in disruption and long delays for those travelling by car into the city.
One of the protesters called Dan, from Llandeilo said they are taking action today because the government and big business "are not moving away from fossil fuels fast enough."
"We are ordinary people in Wales taking a stand, lying down. There is no future in fossil fuels," he added.
A spokesman for Extinction Rebellion, Pembrokeshire said they not involved in the on-going protest, but that the group supports the protesters efforts.
Traffic monitors are showing that Axton Hill in Pembrokeshire is closed because of the demonstration.
This closure is affecting traffic between Angle and Hundleton.
A spokeswoman for Dyfed-Powys Police said: "The protest is currently peaceful but is causing significant disruption to local traffic at the Valero site. No arrests have been made.
Police officers are at the scene and working with partner agencies to minimise disruption to communities and ensure the safety of everyone involved.