At the time of the shooting, Suthin Tharathin was addressing a crowd from the top of a truck, trying to shut-down an advance polling center ahead of the controversial planned February 2 general elections. He later died at a hospital.
Anti-government protesters, led by the Democrat Party, are seeking the removal of Prime Minister Yingluck Shinawatra, whom they accuse of acting on behalf of her brother, ousted former Prime Minister Thaksin Shinawatra. Protesters are also seeking an over-haul of Thailand's political system, which they allege is undemocratic and corrupt.
The Election Commission advised the Thai government to postpone the general elections due to the potential for disruption and violence. The Thai government maintains that the voting will proceed as planned on February 2.
The U.S. Department of State express its concern Sunday about both the opposition's election disruptions and political violence.
"The United States is deeply troubled by efforts to block polls and otherwise prevent voting in Thailand, and by the most recent acts of political violence. While we do not take sides in the political dispute and strongly support freedom of expression and the right to peaceful protest, preventing citizens from voting violates their universal rights and is inconsistent with democratic values.
"We reiterate our call for all sides to refrain from violence, exercise restraint, and commit to sincere dialogue to resolve political differences peacefully and democratically."
Anti-government protests have been ongoing since November 2013.