Just Stop Oil said its supporters blocked access to the Nustar Clydebank facility in West Dunbartonshire by climbing on top of tankers and locking on to the entrance at around 4am on Tuesday.
Others entered the oil terminal, where 12 protesters are sitting on pipes and three are on the silos to halt operations.
The activists said they are taking action in support of their demand that the UK Government ends new oil and gas projects in the UK.
It is the first action its kind in Scotland since the Just Stop Oil coalition began blockading fuel terminals south of the border on April 1, which has seen more than 1,000 arrests.
Neil Rothnie, 69, a retired offshore oil and gas worker from Glasgow, said: “North Sea oil and gas does not offer energy security. The North Sea oil and gas industry has one priority and it is not the climate crisis. It’s not the future of North Sea oil and gas workers.
“And it’s certainly not whether the poor can stay warm. If the Government was serious about a just transition, we would be seeing it here in Scotland.
“Where are the turbine factories in Scotland? Where are the yards building platforms for offshore wind? Where are the projects to properly insulate our houses? When will we get free public transport?”
A Police Scotland spokesman said: “Officers are currently engaging with a group of people who are carrying out a protest at Rothesay Dock in Clydebank, which was reported to police around 4.10am on Tuesday 3 May 2022.
“Police Scotland is a rights-based organisation that puts our values of integrity, fairness, respect and a commitment to upholding human rights at the heart of everything we do.
“This means that we will protect the rights of people who wish to peacefully protest or counter-protest, balanced against the rights of the wider community.”
The UK Government has been approached for comment.