Police arrested at least 40 protesters opposed to shale gas development in easternmost Canada on Thursday after vehicles were set ablaze and a shot was fired.

Authorities moved in to enforce a court injunction against the protesters, who have been camped out on a New Brunswick highway near the town of Rexton for weeks.

The roadway is used by SWN Resources Canada vehicles in regional gas exploration.

The Royal Canadian Mounted Police said they were "met with resistance by some at the site."

"At least one shot was fired by someone other than police, Molotov cocktails were thrown at police and at least five RCMP vehicles were destroyed by fire," they added.

The RCMP is also investigating suspected explosive devices at the scene.

The protesters arrested face dozens of charges, including firearms offences, uttering threats, intimidation, mischief and for refusing to abide by a court injunction.

Shale gas exploration opponents warn of environmental damages from the process known as "fracking" to extract the gas.

Supporters say fracking concerns are exaggerated