A dead horse and two dead cows were dumped in Brighton as thousands of hunt supporters gathered to protest at the Labour party conference.
The dead horse was dumped close to Brighton railway station
Two men were arrested after two calves were left on a path close to a fountain on the Old Steine at 1125 BST.
The horse was left outside Brighton railway station at about the same time.
The RSPCA called it an "appalling act" and the Countryside Alliance condemned the dumpings which were within a mile of the protest at the Brighton Centre.
Public order offences
RSPCA head of press Ann Grain said: "This is a mindless and appalling act which serves no purpose.
"Whoever has done this cannot have any respect for animals."
The RSPCA said Countryside Alliance banners and a handwritten message to Tony Blair were found with the carcasses.
"Given that Countryside Alliance banners were found at the scene, we call on the organisation's leaders to immediately condemn this despicable act," Miss Grain said.
'Risk to health'
Simon Hart, Countryside Alliance chief executive, said: "The alliance totally condemns this action, which will anyway only serve to diminish the message of determination from the massive lawful demonstration outside the conference centre."
The animals were found as thousands of Countryside Alliance campaigners gathered to lobby Labour party members.
Forensic scientists and uniformed police officers began inquiries while council workers were called in to move the decomposing corpses.
Sussex Police said two men had been arrested over public order offences when officers stopped a van in central Brighton, shortly after the animals were found.
An onlooker said the calves were dumped by two men, accompanied by a television crew.
He said: "They arrived in an old van and dragged the animals outside, leaving them on a path in front of a fountain.
"I think the men were quickly arrested by police.
"It's terrible for the people around here and must be a risk to public health.
"I hope they don't carry on acting like this because it won't win them any friends or help their argument."
The pro-hunt campaigners gathered outside the Brighton Centre and marched along the seafront in protest at government plans to ban fox hunting.
Their protest was timed to coincide with prime minister Tony Blair's speech to the conference.