Australian authorities have started the controversial killing of about 400 kangaroos on the outskirts of Australia's capital of Canberra, animal rights activists said Monday.

Between 20 to 40 kangaroos were tranquilized and then killed with lethal injections by defense contractors, said Angie Stephenson, project manager of Animal Liberation New South Wales. She said a baby kangaroo was trampled and killed when the animals were rounded up into a pen.

The Defense Department said last week it would begin killing the eastern gray kangaroos at the abandoned military site outside Canberra after a proposal to truck them to remote forest land proved too costly.

Authorities say the overabundant kangaroos are crowding out other native species, but many animal rights activists are against the killing of Australia's beloved national symbol.

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Stephenson said there were a handful of activists at the site Monday to witness the killing, and that dozens more were expected to gather in the coming days.

The issue has split Australians over the merits of killing kangaroos to protect rare lizards and insects that share their grassy habitat.

Because of the suburban environment, contractors are using tranquilizer darts to catch the kangaroos, then kill them with lethal injections.

Scientists point out that eastern gray kangaroos are abundant and destroying the native grassland habitat of threatened species such as the grassland earless dragon lizard, striped legless lizard, golden sun moth and perunga grasshopper.

European settlers built Australia's cattle and sheep industries on grass seed imported from Britain, and native grassland, which is imperative for some species, is now rare. In some parts of Australia, it can only be found in old cemeteries where livestock never grazed.