Members of a Russian environmental group say masked men attacked their leader in the southern Russian city of Krasnodar late Thursday.
Andrei Rudomakha, head of Environmental Watch of the North Caucasus, was hospitalized with multiple injuries including a fractured skull and broken nose.
Rudomakha and several other activists were returning from a trip to Russia's Black Sea region, where they had documented the illegal construction of a luxury mansion.
Local authorities said they are investigating the incident.
For more than 20 years, Environmental Watch has exposed illegal landfills, the destruction of landscapes and the contamination of waterways in Russia's south - the Krasnodar, Stavropol, Rostov, Adygea, Karachayevo-Cherkesia, and Kabardino-Balkaria.
Some of the group's investigations have exposed land grabs by Russian local officials.