Wangari Maathai, the first African woman to win the Nobel Peace Prize for her campaigns to save Kenyan forests, died in hospital on Sunday after a long struggle with cancer.

Maathai, 71, founded the Green Belt Movement in 1977 to campaign for tree planting and prevent environmental and social conditions deteriorating and hurting poor people - especially women - in rural Kenya.

Her movement expanded in the 1980s and 1990s to embrace wider campaigns for social, economic and political change, setting her on a collision course with the government of the then-president, Daniel Arap Moi.

Maathai, who won the Peace Prize in 2004, had to endure being whipped, teargassed and threatened with death for her devotion to Africa's forests and her desire to end the corruption that often spells their destruction.

"It's a matter of life and death for this country," Maathai once said.

"The Kenyan forests are facing extinction and it is a man-made problem. You cannot protect the environment unless you empower people, help them understand that these resources are their own, that they must protect them."

Maathai was born in the central highlands of Kenya on April 1, 1940. She earned a master's degree in the US before becoming the first woman in Kenya to receive a doctorate for veterinary medicine and be appointed a professor.

In 1989, Maathai's protests forced Moi to abandon plans to erect an office tower in Uhuru Park, an oasis of green that flanks the main highway running through the centre of the Kenyan capital Nairobi.

"It is a sad moment for myself and this country at large," said Nairobi resident Gikonge Mugwongo.

"We have lost a serious personality who shaped not only Kenya, but the world at large. We have lost a great mind, a great woman who could change lives in this country."

In 1999, Maathai was beaten and whipped by guards during a demonstration against the sale of public land in Karura Forest in Nairobi.

The forest is home to wildlife such as duiker antelopes and civets, and caves used by Mau Mau fighters in their 1952 struggle against British rule.

"To interfere with [the forest] is to interfere with the rain system, the water system and, therefore, agriculture, not to mention the other industries dependent on hydroelectricity," Maathai said.

Her movement spread across Africa and has gone on to plant more than 47million trees to slow deforestation and erosion. She joined the UN Environment Programme in 2006 to launch a campaign to plant one billion trees worldwide.

"Her departure is untimely and a very great loss to all of us who knew her - as a mother, relative, co-worker, colleague, role model and heroine - or those who admired her determination to make the world a peaceful, healthy and better place for all of us," her movement said.

Tributes poured in for Maathai on social media from around the world.

"She has left a lasting legacy in greater awareness and work in protecting our environment and the world," said Nelson Mandela Foundation CEO Achmat Dangor.

"It was with great sadness that we learned today of the passing of this exceptional environmental activist. We offer our deepest condolences."

Dangor said he was honoured to have Maathai deliver the annual Nelson Mandela lecture in 2005.

He quoted an excerpt of her speech delivered at the lecture: "We need people who love Africa so much that they want to protect her from destructive processes.

"Practise the 3R campaign [reduce, re-use, recycle], get involved in local initiatives and volunteer your time for services in your community."

Kenyan politician Martha Karua said on her Twitter account: "We join family and friends in mourning Prof Wangari Maathai, a phenomenal woman, a friend and role model. You lived, you inspired."

Maathai was elected to parliament in 2002 and appointed deputy minister for the environment in 2003.

"Rest in peace Dr Wangari Maathai. A great woman, an inspiration for many women across Africa, a magnificent visionary and embodiment of courage," Tanzanian President Jakaya Kikwete said in a Twitter message.