Blood flowed from beneath a shoreside tent as dolphins were killed at Taiji, Japan, on Tuesday.
The waters made famous by the Oscar winning documentary The Cove ran red in the killings by local fishermen, who worked for four days on a herd of 250 bottlenose dolphins held in nets.
Activist Melissa Sehgal, of Sea Shepherd's Cove Guardians, said in a livestream from the scene that 52 of the dolphins were separated to be trained for marine park work, but many of the remaining dolphins were killed for their meat.
Ms Sehgal described the scene as dozens of dolphins were herded or dragged into the tent, later to emerge stacked on skiffs for transfer to the town's butcher house.
The processing, which began last weekend had been horrific to witness, she said.
One group of dolphins had been left alive, potentially to be driven out to sea again.
"We can't say for sure until we see them being driven out," Ms Sehgal said. "But we're trying to remain hopeful."
A renewed spotlight fell on the long-standing hunt when US ambassador to Japan Caroline Kennedy said she was deeply concerned at the inhumaneness of drive hunt dolphin killing.
British ambassador to Japan Tim Hitchens joined condemnation of the hunts which he said caused terrible suffering.
Australian Environment Minister, Greg Hunt, told Sky News: "It's not something I'm comfortable with. It's not something that I respectfully support.”
Japan's Chief Cabinet Secretary Yoshihide Suga told a news conference that dolphin fishing in Japan was carried out appropriately in accordance with the law.
"Dolphin fishing is a form of traditional fishing in our country," Mr Suga said, responding to a question about Ms Kennedy's criticism. "We will explain Japan's position to the American side."