It is with great sadness that the family of Kenyan environmental activist Wangari Maathai announces her passing away on 25th September 2011 at the Nairobi hospital after a prolonged and bravely borne struggle with cancer," said a statement issued on Monday via the Green Belt Movement she founded.

Maathai became a key figure in Kenya after founding the movement in 1977, staunchly campaigning for environmental conservation and good governance.

She won the 2004 Nobel Peace Prize for her reforestation work in her native Kenya -- the first African woman, the first Kenyan and the first environmentalist to receive this honour. Her organisation has planted some 40 million trees across Africa.

The first woman in east and central Africa to earn a doctorate, Maathai also headed the Kenya Red Cross in the 1970s. Aside from her conservation work, Maathai was elected an MP in 2002 and then named the environment assistant minister, a position she held between 2003 and 2005.

For more than a decade from the 1980s, her movement also joined the struggle against the dictatorial regime of Kenya's former president Daniel Arap Moi, with Maathai repeatedly teargassed and beaten by police.

During that time, she famously campaigned against the construction of a high-rise building in a park in central Nairobi, stopped the grabbing of forest land outside the city and successfully pressed for the release of 51 political prisoners.

The award-winning Maathai in recent years founded green groups and launched several campaigns against climate change and environmental protection. Outside Kenya, Maathai was involved in efforts to save central Africa's Congo basin forest, the world's second largest tropical forest.

Maathai, who was divorced, is survived by three children and a granddaughter.

"Professor Maathai’s departure is untimely and a very great loss to all who knew her as a mother, relative, co-worker, colleague, role model, and heroine, or who admired her determination to make the world a more peaceful, healthier, and better place," said the statement.

Tributes poured in for her campaigning work from Kenya and abroad. "It is a deeply distressing and painful loss, not only to the environmental movement globally but also to the social justice movement," Kumi Naidoo, executive director of Greenpeace International, told the BBC. "She was an inspiration to so many of us, far beyond the African continent."

Comments on a Facebook page set up to mourn her death said her "innovativeness and steadfastness will be greatly missed."

Maatheri was born in a "small village ... but proceeded to leave a mark in the global arena," read one comment. She was "the foremost activist of environmental rights in Kenya. She served her country diligently," another added.