Wangari Maathai won the Nobel Peace Prize in 2004 for promoting women's rights, conservation and transparent government.
She served as an MP in the Kenyan government and started up the Green Belt Movement in the late 70s, which has encouraged schools, church groups, government offices and ordinary people across Africa to plant 20-30 million trees.
She died in Nairobi early yesterday morning (Monday) aged 71. She had been having treatment for cancer for a while.
"It is with great sadness that the Green Belt Movement announces the passing of its founder and chair, Prof Wangari Muta Maathai, after a long illness bravely borne," the Green Belt Movement said in a statement on its website. "Her loved ones were with her at the time.
Ms Maathai became known globally for campaigns against government-backed forest clearances in Kenya in the late 1980s-90s. She was also the first woman from east and central Africa to get a doctorate, in veterinary anatomy.
"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," the Green Belt Movement added.
During former president Daniel Moi's long rule, she was beaten, tear-gassed and whipped as she protested in the streets against environmental damage around Nairobi through the 1980s and 1990s. But when Mr Moi left power in 2002, Mrs Maathai was elected to parliament and became assistant minister for environment in President Mwai Kibaki's first government in 2003.
Yesterday tributes flooded in on Twitter, Facebook and morning radio call-in shows in Kenya. "Rest in peace Dr Wangari Maathai. A great woman, an inspiration for many women across Africa, a magnificent visionary and embodiment of courage," said Jakaya Kikwete, Tanzania's president on Twitter.
The President of Liberia, Ellen Johnson-Sirleaf, said: "Africa, particularly African women, have lost a champion, a leader, an activist. We're going to miss her. We're going to miss the work she's been doing all these years on the environment, working for women's rights and women's participation," she said.
Accepting the Nobel Prize in 2004, Ms Maathai said she hoped her success would motivate other women to take on a more active role in the community.
"I hope it will encourage them to raise their voices and take more space for leadership," she said.