A truck carrying the pigs overturned in an accident and was then stormed by a mob, who then killed the pigs for meat.
The Society for the Prevention of Cruelty to Animals (SPCA) says police did nothing to stop the crowd and then loaded carcasses into an unmarked van.
A police spokeswoman says if true, such actions would be unacceptable.
"Animals were gutted while they were still alive," the South African Press Association quotes SPCA spokesperson Annette Rademeyer as saying.
"They were stabbed and their hind quarters hacked at - it's the most horrific savagery I've ever experienced," she said as an investigation into the incident began.
According to South Africa's Cape Times, the SPCA say police failed to help them stop the killing spree near King William's Town on Sunday.
"Just when we thought things couldn't get worse, some members of the police deliberately removed number plates from official vehicles and loaded animals into the vans to steal them," Ms Rademeyer said.
The SPCA said that it put down 47 of the 150 pigs on board the truck and at least 30 others were killed.
"We regard this in a very serious light and it will receive attention at a very high level," Eastern Cape police spokesman Marinda Mills is quoted as saying.
Last Mod: 07 Ağustos 2007, 16:30