In the environmental documentary The Cove, diver Mandy-Rae Cruickshank swims with dolphins near a Japanese cove where the mammals are hunted and slaughtered. (Maple Pictures)The struggle to release the documentary The Cove in Japan has turned into a fight over freedom of speech, with a prominent group urging theatres to screen the Oscar-winning dolphin-hunt exposé.

An assembly of Japanese filmmakers, journalists and academics have signed a letter calling on theatres to screen the film and not back down amid threatened protests from nationalist groups, who have criticized the doc as anti-Japanese.

Approximately 600 people crammed into a conference hall in Tokyo on Wednesday for a rare screening of The Cove. Queues formed hours before the scheduled start time for the sold-out show — the first major screening of the movie in Japan since its debut at last fall's Tokyo International Film Festival.

There were so many interested in the film that the excess spilled out into the hall's lobby, with several dozen people watching via a video feed outside the hall.

Though more than two dozen uniformed and plainclothes police officers were stationed outside the hall, there were no disruptive demonstrations. However, a few people outside quietly handed out flyers calling for the film to be banned.

Hiroyuki Shinoda, chief editor of Tsukuru magazine and one of the organizers of Wednesday's screening, urged those in attendance to contact theatres still mulling whether to release The Cove and ask that it be shown.

"I want people who gathered here tonight to send supportive messages to movie theatres," Shinoda said, adding that the protesters who had turned up didn't follow through on threats to "do anything harmful."

He also encouraged Japanese media to "stand more strongly for freedom of expression."

Earlier this month, several cinemas cancelled their planned theatrical release of The Cove, after right-wing protesters decried the film's graphic depiction of the annual dolphin hunt and slaughter in the southern Japanese village of Taiji.

Marine mammal specialist Ric O'Barry is a dolphin trainer turned environmental activist. (Maple Pictures) Though few Japanese knowingly eat dolphin meat, which carries high levels of mercury, some consider hunting the mammals (as well as whales) a cultural tradition.

Directed by Louie Psihoyos, The Cove was the toast of the 2009-2010 film festival and movie awards season and won myriad honours, including the prestigious Academy Award for best documentary in March.

Ric O'Barry, a former dolphin trainer for the Flipper TV series who has since become a dolphin activist, made a surprise appearance at Wednesday's screening in Tokyo.

"Those [film] awards are given for entertainment value, and for that reason alone the Japanese people should be able to see it and make up their own mind," said O'Barry, who is a central character in The Cove.

With files from The Associated Press