Nalcor Energy says around 50 protesters have broken into the Muskrat Falls hydroelectric site in Labrador.
Nalcor spokeswoman Karen O'Neill says protesters and several vehicles breached the main gate Saturday afternoon.
She says a blockade of around 150 people outside the gate has obstructed access to the work site near Happy Valley-Goose Bay.
Protesters have blocked access to the site over the last week, despite a court injunction that led to nine arrests Monday.
The actions are in response to Nalcor's confirmation that methylmercury levels are expected to rise in the reservoir created by construction of the project, raising health concerns.
Nalcor has said flooding is to begin later this month, though the company tweeted Saturday it won't occur this weekend.
O'Neill says Nalcor is working to secure the site and protect the safety of "people, facilities and equipment."
The RCMP announced Saturday that the main highway to Muskrat Falls was closed.
Police didn't say if it was linked to the protests, only citing public safety concerns.