Bob Hunter, 63, the co-founder of the Greenpeace environmental group, has died after battling prostate cancer.
Canadian Bob Hunter (left) took an active part in Greenpeace's work
He helped to launch Greenpeace in 1971 and went on to draw global attention with campaigns against nuclear testing and pollution of the world's oceans.
Mr Hunter was also well known for his work as a journalist and author.
"Bob was an inspirational storyteller, an audacious fighter and an unpretentious mystic," Greenpeace Canada chairman, John Doherty, said.
"He was serious about saving the world while always maintaining a sense of humour."
Mr Hunter's passion for ecology and approach to communications helped to define Greenpeace, the group said.
He was responsible for adopting the term "Rainbow Warriors" to describe Greenpeace activists, which also became the name of the Greenpeace ship.
He brought public attention to the hunting of whales and seals.
"I felt like the first time I met him I had seen a unique genius. He seemed to see things that other people missed," former Greenpeace director Rex Weyler told the BBC's World Today radio programme.
"I remember him saying things like, 'Ecology is going to be the biggest revolution in human history. It's going to change everything. It's not just a matter of cleaning up rivers and oil spills, but it's going to change science, politics and philosophy'.
"He was such a grand thinker. He saw this environmental movement in the 1970s when no such thing existed.
"The great thing about him was that he made it happen."
Greenpeace now has 2.5 million members in 40 countries.
Time magazine named Bob Hunter one of the 20th Century's top eco-heroes.
In other roles, he worked as an environmental reporter on TV as well as hosting Paper Cuts - in which he commented on the day's newspaper headlines clad in a bathrobe.
"This was a man with a great loving heart, a brilliant mind and a massive spirit," said Stephen Hurlbut, CityTV vice-president of news programming.
"Bob Hunter changed our world. It is a sadder world today, but a better world because of him."
He died attended by his wife and three children, Canadian media report.