Award-winning Honduran indigenous rights activist Berta Caceres has been killed in the western town of La Esperanza.
Two men entered Ms Caceres' home on Thursday and shot her dead, Hugo Maldonado, of the Honduran Committee for the Defense of Human Rights, said.
Her mother said Ms Caceres had been killed "because of her struggle" for indigenous and environmental rights.
Ms Caceres had told police she had received death threats.
A member of the Lenca indigenous group, she was one of the founders in 1993 of the National Council of Popular and Indigenous Organizations of Honduras (COPINH).
In 2015, she was awarded the Goldman Prize, one of the most prestigious awards for grass-roots environmentalism.
The judges said she had "rallied the indigenous Lenca people of Honduras and waged a grassroots campaign that successfully pressured the world's largest dam builder to pull out of the Agua Zarca Dam".
The dam would have flooded large areas of land and cut off the supply of water, food and medicine for hundreds of Lenca people.
The United Nations' Special Rapporteur on the Rights of Indigenous Peoples, Victoria Tauli-Corpuz, said she had heard the news of Ms Caceres's "dastardly" killing with "deep sorrow".
"It is highly probable that her assassination is linked with her work in protecting the human rights of the Lenca indigenous peoples to their lands and territories," Ms Tauli Corpuz said.
Ms Caceres's brother Gustavo said she had always confronted things "head on" but that she had never carried a weapon because "her voice was her weapon".
He asked Honduran President Juan Orlando Hernandez to investigate her murder and punish those responsible.
Honduras has one of the highest murder rates in the world.
Ms Caceres' mother said police had not ruled out a robbery gone wrong as the motive for the activist's murder, but she said "we all know it was because of her struggle".