Queen guitarist Brian May has led several hundred black-and-white-clad demonstrators through the streets of London, urging the government to scrap plans to cull thousands of badgers.
The cull, which started in two areas of southwest England on Saturday, will see up to 5,000 badgers killed in a bid to combat tuberculosis in cattle.
But animal welfare groups are outraged by the plan, claiming it is inhumane and will do little to wipe out the disease.
Bovine TB spreads from badgers to cattle, and farmers are forced to slaughter thousands of infected animals every year.
But May, 65, said there was "not a shred of reason" for the culls in Gloucestershire and Somerset in southwest England, and delivered a 235,000-signature petition against it to Prime Minister David Cameron's office at Downing Street.
"It is a very good time for Cameron to reconsider and withdraw from this monstrous cull, in the public interest," the rock star said as he joined hundreds of animal rights activists in central London.
The pilot schemes will see up to 70 per cent of badgers killed in Gloucestershire and Somerset.
If successful, the government plans to roll out the cull in other rural areas hit badly by bovine TB.
The government argues that the cull is necessary to stop the spread of the disease, which forced English farmers to slaughter 28,000 cattle last year.
"There's not a single country in the world that has successfully borne down on bovine TB without doing something about the reservoir in the wildlife population," junior environment minister David Heath told BBC radio.
"Whether it's Ireland, New Zealand, Australia, all the countries that have successfully dealt with this disease have employed a policy very similar to ours."
But the protesters say the cull, involving trained marksmen shooting the badgers, is unnecessarily cruel.