Wildlife activists are preparing to become human shields to prevent a cull of more than 400 kangaroos in Australia's capital city in a slaughter that has drawn international condemnation.

The eastern grey kangaroos, an example of which features on the country's coat of arms, will be herded into pens, shot with tranquilliser darts and killed by injection after the Australian Capital Territory government refused to allow them to be relocated.

Contractors will be ready within days to destroy the marsupials on a military site in northern Canberra to protect endangered flora and fauna from overgrazing. The authorities have turned down an alternative plan to remove the kangaroos, citing expert advice that it would be inhumane.

Pat O'Brien, the president of the Wildlife Protection Association of Australia, said people were “incredibly angry” and that protesters were prepared to be arrested in front of highsecurity fences surrounding the 116-hectare (286-acre) site. “We are going to blockade the gates 24 hours a day from Saturday so they can't get in and kill them,” Mr O'Brien said. “I'm sure there will be people standing in front of the dart guns.” The cull has brought two ageing rock stars into conflict. Sir Paul McCartney, the former Beatle, called for an end to “this shameful massacre” while Peter Garrett, the Environment Minister and former frontman of Midnight Oil, the rock band, said that culling programmes were sometimes necessary.

Viva!, the British animal welfare group backed by Sir Paul, has joined local activists in demanding another review of the option to relocate the animals. It says that people in 36 countries have lodged an online petition, reflecting an international outcry in 2004 when about 900 kangaroos were destroyed at the site of a dam supplying water to Canberra. “There is an urgent need for action to protect kangaroos from a barbaric industry, which slaughters them for meat and leather,” Sir Paul says in a statement on the vegetarian group's website.

Environmentalists have also accused the Government of hypocrisy in light of its vocal opposition to whaling by Japan. Australia has led efforts this month within the International Whaling Commission to close the legal loophole that allows “scientific” killing.

The minister, a former leader of Australia's conservation movement, said that the cull would benefit the local environment and wildlife in the long term. It is being carried out to protect rare grasslands and the threatened perunga grasshopper, golden sun moth and ginninderra peppercress, a perennial herb.

“Australians care a great deal about their environment and about their wildlife,” Mr Garrett said. “But when there are significant imbalances and the possibility that you'll have conditions which don't benefit the environment and wildlife in the long term, the programmes like this - humanely and properly administered - are sometimes necessary.”

Permits are issued each year for the slaughter of millions of kangaroos to protect the environment, giving rise to a A$200 million (£92 million) industry in skin and meat exports. The animals' numbers have increased dramatically since European settlement because of additional water for domestic stock.

The population is at a historical high of about 60 million, despite the commercial kangaroo harvest of about 3.5 million last year. This makes them among the most common large wild land mammals on Earth.

Times Recommends

• UN may halt 400-year-old 'freedom of the seas'
• Where the Rosetta belongs can’t be set in stone
• Gas deal on hold, but Ukraine may pay in the end