The action took place just west of Panama City, on the Pan-American Highway that runs from south to north through South, Central and North America.
Several reporters and photographers were on site to cover the event when the man got out of his car and shot two of the activists at close range.
The ongoing protests are against the operation of a large open pit mine in a species-rich jungle area. Canada’s First Quantum Minerals was recently granted a permit to mine copper for another 20 years, with an option to extend for another 20 years after that.
The arrested person is identified as a 77-year-old retired lawyer with citizenship of both the United States and Panama.