The group - called Block Lockheed - moved on to the access road at 5.45am today (Monday) and have used 'lock-ons' to prevent access to the 64 acre site
The protesters said they plan to continue to block the defence firm until it ceases operations.
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Rebekah Lloyd, spokesperson for Block Lockheed, said: "Lockheed Martin get rich selling weapons that cause untold destruction to lives in Palestine and Yemen, whilst simultaneously destroying our environment.
"This is a line in the sand, we are telling them we won't accept it anymore. We stand with our friends across the world campaigning for an end to this blood-soaked industry."
The US armoured fighting vehicle plant currently employs around 900 people at its Ampthill facility.
Lockheed Martin declined to comment when Bedford Today contacted them.
Bedfordshire Police's chief superintendent David Boyle said: “We are aware of a protest in Hazelwood Lane, Ampthill, and are currently at the scene working with organisers and businesses to minimise disruption.
“We respect people’s right to protest in a peaceful and responsible way, and we will continue to work with everyone to ensure public safety.”