Berta Caceres, who won the 2015 Goldman Environmental Prize, has been shot dead at her home in the town of La Esperanza.
At least 10 people have been shot dead by men dressed in police uniforms in an attack in the Honduran capital of Tegucigalpa, security and hospital forces said.
Five men arrived in a vehicle at a billiards hall in the violent northeastern December 1 neighbourhood and opened fire with AK-47 assault rifles.
The men wore police clothing when they attacked, killing eight people and injuring others, said a security ministry source who asked not to be named.
Two other people later died at the Hospital Escuela Universitario, hospital spokesman Miguel Osorio said.
Police said the pool hall was controlled by the Barrio 18 gang and there were about 30 people in it at the time. No arrests have been made.
The massacre took place a year after a similar shooting in a nearby pool hall left five people dead.
There are 16 homicides on average in Honduras every day, mostly due to drug cartels and territorial disputes by violent gangs dedicated to extortion.