A GREENPEACE activist trying to free tuna from a commercial fishing net in the Mediterranean was harpooned through the leg by fishermen on a French boat, the environmental group said.

"In a non-violent action, Greenpeace activists in zodiac boats attempted to lower the side of a purse seine net with sand bags to free the fish," said Isabelle Philippe of Greenpeace France.

"At that moment, fishermen violently attacked the activists, harpooning one of them through the leg," she said.

The injured man, a British national identified by the group as Frank Huston, was transferred to a hospital in Malta where he was to undergo surgery on Friday evening (local time).

"His life is not threatened, but he is in serious condition," Ms Philippe said.

The fishing ship, the Jean-Marie Christian VI, was one of several French tuna vessels in the area when the attack occurred in international waters near Malta, she added.

Several boats surrounded the zodiacs, threatening them with knives attached to long poles and some of the fishermen also fired flare guns at a Greenpeace helicopter hovering overhead to monitor, Greenpeace said in a statement.

A French navy vessel was seen arriving in the area after the clash had ended, the statement added.

Greenpeace said that it had stationed two ships in the Mediterranean, the Rainbow Warrior and Arctic Sunrise, to confront tuna fishing boats during the short tuna fishing season.

Pascal Husting, director of Greenpeace France, condemned the fishermen for their violent response.

"The tuna fishermen only know brutality: all our militants have are bags of sand to weigh down the nets and free the fish," he said.

Bertrand Wendling, head of Sathoan, which represents the owners of many of the French tuna fishing boat including the one whose nets were targeted by Greenpeace, accused the group of interfering with a legal business activity and jeopardising the livelihoods of ordinary fishermen.

"Red tuna fishing is a highly regulated, legal and permitted activity," he said.

"It is out of the question that you go out on the high seas to stop people working when they have only 15 days in which to earn a living, because the season runs from May 15 to June 15 and until now the boats have been completely stuck."

About 100 fishing vessels navigate the Mediterranean during the short tuna fishing season.

Many of the boats carry net cages used to encircle the tuna swarms, which are then towed offshore to be fattened and shipped in giant freezer ships to Japan, where it is a mainstay of sushi and sashimi.

Industrial-scale fishing and harvesting on the high seas has caused stocks to plunge by up to 80 per cent in the Mediterranean and eastern Atlantic, where they come to spawn in the warmer waters.

Earlier this year the European Union and the United States backed an international trade ban on tuna fished from these waters, but Japan lobbied successfully and the proposal was defeated.

