Protestors from a group calling itself Animal Freedom Movement (AFM) occupied the Marshall BioResources (MBR Acres) dog breeding facility at Wyton, near Huntingdon.
Protestors say several arrests have been made already.
The group of 25 protestors are demanding that MBR shut down the facility and rehome the dogs, whilst the British Government commits to phasing out all animal testing and the sale of animal-tested products by 2025.
The group entered the facility early this morning, and unveiled a banner reading "End Animal Testing."
AFM has said it has two demands, firstly for MBR to immediately shut down the facility and rehome all the dogs currently there.
Secondly, they demand the British government urgently phase out animal testing, and the sale of animal-tested products, by 2025.
They plan to occupy the site until commitment towards their two demands is made by MBR and the Government.
Dillon Parsons, spokesperson for AFM, has said: "Using animals for testing is not only cruel, inhumane, and barbaric, it is completely unnecessary.
"There is no need for nearly three million painful and torturous tests on animals in the UK each year when we have technology that is more reliable and efficient.
"The UK claims to be a nation of animal lovers, we demand The Government and MBR show that to be true"
"Whilst many of us console ourselves with the idea animal testing serves a greater purpose, a parliamentary debate revealed that 90 per cent of tests on animals are ultimately useless."
Rosa Sharkey, who is among those inside the facility, added: "The science is clear, our morals should be too, an end to animal-testing is the only step forwards for the industry.
"MBR must do the right thing by closing its Cambridgeshire facility and rehoming the puppies they intend to send for death. The British Government has a clear mandate from its people to end the barbaric practice of animal testing."
AFM is urging anyone who supports an end to animal testing to attend a peaceful demonstration outside The Home Office on Monday June 27 at 4.30pm.
Cambridgeshire Police and MBR Acres have been approached for a comment.
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