In a post to social media, Extinction Rebellion Norway said eight kayaks and three motorboats had entered a security zone outside the Karsto processing plant to "prevent the normal operation of tankers."

Activists, including Thunberg, also gathered on land to block the facility's gates.

"We cannot stand idly by as the fossil fuel industry robs people of their lives and destabilises our life support systems," Thunberg was quoted saying in the post.

The Karsto plant is operated by Norway's Equinor and is the largest of its kind in Europe, according to the company.

According to Equinor, around 30 oil and gas fields are connected to the plant. A company spokeswoman confirm the protest had been started.

"A group of activists have gathered outside parts of the facility, but at this stage the plant is operating as normal," Ellen Maria Skjelbaek told AFP.

Skjelbaek said police were dealing with the protest.