The outcry has been growing against the hunt in Taiji, western Japan, since an award-winning American documentary "The Cove" this year showed dolphins being herded into an inlet and killed by fishermen with spears.
The Save Japan Dolphins Coalition, led by Ric O'Barry, the dolphin trainer for the 1960s "Flipper" TV series, said on Tuesday that group representatives witnessed and filmed 70 bottlenose dolphins - the same kind as Flipper - being released Sunday.
Taiji and its fishing association declined comment. The hunt kills about 2,000 dolphins a year, and residents say it is part of their tradition and a way of getting food.
The released dolphins were part of a catch of about 100 on September 9. The Taiji fishing association had said it would sell about half to aquariums and set the rest free.
The town has said it is unclear if it will continue to release dolphins.
"The world is watching," said O'Barry, who visited Taiji earlier this month. "Stopping the slaughter and sale of dolphins would be a major victory for the people of Japan."
He said dolphin meat is contaminated with dangerous levels of mercury.
The Japanese government has issued warnings about pregnant women eating dolphin meat, but says it is safe in small quantities.
Taiji fishermen on September 9 also caught 50 pilot whales, which were killed and sold as meat.
David Phillips, director of the Earth Island Institute, a California-based environmental group, said a member is in Japan to watch whether Taiji kills any dolphins.
"We will expand vigilance in Taiji and bring greater world attention to ensure that the dolphins are released and that the slaughter does not resume," he said.
"The Cove" has won more than a dozen awards, including the audience award at the Sundance Film Festival. It has yet to be released in Japan.
