The hunt, in which pods of dolphins are driven into the cove, became controversial in 2009 with the release of a documentary, "The Cove." Some of the dolphins are selected for sale to aquariums and others slaughtered for food, CNN reported.
The Sea Shepherd Conservation Society, a U.S. group dedicated to protecting marine life, has been monitoring the hunt. The group said in a tweet Friday five pods of dolphins had been driven into the cove, where they are to be kept until Sunday..
The experience is "violent and stressful" for the animals, Sea Shepherd said.
"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," the group said.
Taiji, on the Pacific coast about 50 miles south of Osaka, has been a center of Japanese whaling since the 17th century.
Wayakama Prefecture officials issued a statement accusing environmentalists of "psychological harassment" and saying Taiji 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."
U.S. Ambassador to Japan Caroline Kennedy said in a tweet she was "deeply concerned" about the dolphin hunt, adding that the U.S. government "opposes drive hunt fisheries."