Story highlights More than 250 dolphins have been herded into a cove by fishermen, U.S conservationists say

The bottlenose dolphins are being selected for captivity or slaughter, they say

Local officials have previously defended hunting of dolphins in Taiji, Japan, as traditional practice

U.S. envoy to Japan, Caroline Kennedy, says she's "deeply concerned" by inhumane killing

Japanese fishermen rounded up more than 250 bottlenose dolphins in a secluded cove to kill for meat or sell into a lifetime of captivity, U.S. conservationists warned.

The annual hunting of dolphins at Taiji Cove highlights the rift between conservationists worldwide who see it as a bloody slaughter and Japanese who defend it as a local custom.

The Sea Shepherd Conservation Society first raised the alarm over the plight of the dolphins Friday, saying five separate pods of bottlenose dolphins had been "driven into Taiji's infamous killing cove."

The group warned that the dolphins would "face a violent and stressful captive selection process. Babies and mothers will be torn from each other's sides as some are taken for captivity, some are killed, and others are driven back out to sea to fend for themselves."

By the end of Saturday, 25 dolphins had been removed from their pod and taken "to a lifetime of imprisonment," the group said. One of them died in the process and will be butchered, it said.

The dolphins will be kept penned in the cove for another night before the selection process begins again Sunday.

'Panicked, frightened and fatigued'

The Sea Shepherd Conservation Society live-streamed video of events in the cove Saturday and posted frequent updates on Twitter.

"Killers continue to ruthlessly wrap bottlenose dolphins into nets and drag them to the shore for selection," one update said.

Another, a few minutes later, said, "Panicked, frightened, and fatigued, another portion of the bottlenose pod is driven closer to the shore."

"Killers and trainers tore half of the pod apart today, and will finish tomorrow," was the final post on the day's hunt.

Caroline Kennedy , who was sworn in last year as the U.S. ambassador to Japan, tweeted her condemnation of the process.

"Deeply concerned by inhumaneness of drive hunt dolphin killing," she said. The U.S. government "opposes drive hunt fisheries."

CNN was unable to reach anyone at the town office for Taiji, a community of about 3,000 that juts into the Pacific Ocean, or the local fishermen's union for comment.

But local officials have reacted angrily in the past to Western criticism of what they say is a traditional practice dating back centuries.

Officials say criticism is biased, unfair

A 2009 Oscar-nominated documentary film, "The Cove," brought the issue of dolphin hunting in Taiji to the fore with bloody scenes of dolphin slaughter.

The Wakayama Prefecture, where Taiji is, condemned the film in an online response as distorted, biased and unfair to the fishermen.

"The Taiji dolphin fishery has been a target of repeated psychological harassment and interference by aggressive foreign animal protection organizations," it said.

"Taiji dolphin fishermen are just conducting a legal fishing activity in their traditional way in full accordance with regulations and rules under the supervision of both the national and the prefectural governments. Therefore, we believe there are no reasons to criticize the Taiji dolphin fishery."

The Japanese practice of whale hunting has also put it in conflict with the views of much of the world.

Japan's fleet carries out an annual whale hunt despite a worldwide moratorium, taking advantage of a loophole in the law that permits the killing of the mammals for scientific research. Whale meat is commonly available for consumption in Japan.

Environmental activists warn that dolphin meat, also sold for consumption in Japan, contains dangerously high levels of mercury and other toxins.

'Stop the cruel slaughter'

Celebrities joined in the condemnation of the latest Taiji dolphin hunt via Twitter.

Former Guns N' Roses drummer Matt Sorum , who works with the Dolphin Project, posted in support of Kennedy's message, saying he had been to Taiji himself.

"You could be a world hero if you can put pressure to stop the cruel slaughter in Taiji," he said to Kennedy.

Actress Kirstie Alley tweeted, "HUNDREDS of DOLPHINS awaiting SLAUGHTER in #THECOVE ... JAPAN, STOP THIS HORROR!!"