Isaias Manalo, 23, Anakpawis secretary-general for Mindoro Oriental, died from bullet wounds in the head and abdomen. His companion, was unhurt.
Reports said Manalo was slain by a man who had tailed him and his companion after a meeting with Calapan mayoral bet Bodgie Ignacio.
The militant group Karapatan condemned Manalos killing and accused soldiers based in Mindoro Oriental of having a hand in it.
Karapatan said Manalo was part of a 61-man group which the military held "hostage" inside the Philippine Independent Church in Roxas town two weeks ago. Rene Alviar, Arnell Ozaeta and Benjie Villa