Up to 400 protesters blockaded the entrance to Hinkley Point nuclear power station in Somerset.
They warned this could be the first of a number of demonstrations over plans for a new nuclear power station at the site.
Power firm EDF said of 500 employees at the plant, only essential staff had been called in and had arrived by bus at dawn.
Deliveries had also been re-scheduled.
An EDF spokesman said: "The station's still generating power and essential staff are on site."
Any unexpected deliveries had been taken to an alternative entrance.