NAIROBI, Sept 26 (Reuters) - 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 to prevent environmental and social conditions deteriorating and hurting poor people, especially women, living 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, tear-gassed 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, you inform them, and you 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 United States 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."

"MAGNIFICENT VISIONARY"

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 covers more than 1,000 hectares and is home to wildlife such as duiker antelopes and civets, as well as caves used by Mau Mau fighters in their struggle against British rule.

She called the clearance of forests a "suicidal mission".

"To interfere with them is to interfere with the rain system, the water system and therefore agriculture, not to mention the other industries dependent on hydro-electricity."

Maathai's movement spread across Africa and has gone on to plant more than 47 million trees to slow deforestation and erosion. She joined the U.N. Environment Programme in 2006 to launch a campaign to plant a 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 in a statement.

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

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

Besides founding the Green Belt Movement, Maathai was also 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. (Additional reporting by Reuters Television; Writing by David Clarke; Editing by xxx)