The activists doubted Phnom Penh's capacity to single-handedly build and manage such a vast and risky project.
Santi Choakchaichamnankij, a representative of Bangkok-based Energy Watch, said it would be difficult for Cambodia to construct the nuclear plant, as it would require enormous capital and human resource investments.
"Cambodia itself doesn't need such a large-scale power plant," Mr Santi said. "The project will likely supply electricity to neighbouring countries, including Thailand. The Thai government should make it clear whether it is involved."
The prospect of Cambodia building a nuclear power plant was first raised by Prime Minister Hun Sen at the 30th Asean Ministers Meeting on Energy in Phnom Penh last month.
Hun Sen told the meeting he was interested in building the plant in Koh Kong to pave the way for economic expansion in the country.
But environmental activist Witoon Permwongsacharoen of the Foundation for Ecological Recovery said Cambodia consumed only 1,000 megawatts of electricity per year, meaning there was no need for the country to invest in nuclear power.
Mr Witoon said if Cambodia insisted on going ahead with nuclear energy, Asean members should intervene as the project would impact neighbouring countries.
Wichan Khantuwarn, coordinator of the Network Against Nuclear Power Plant Construction in Trat province, said he did not believe Cambodia would invest in nuclear power, as it had already invested in a coal-fired plant in Koh Kong province.
The province is too small to accommodate both coal-fired and nuclear power plants, he said, warning that the project might face strong opposition from residents in border areas.
Thailand also has plans to develop nuclear energy infrastructure.
Under the Energy Ministry's 20-year power development plan, Thailand will have five nuclear plants with a combined generating capacity of 5,000 megawatts by 2025.
Trat, which borders Koh Kong, is one of the provinces shortlisted as a potential site for a nuclear plant.
If Cambodia goes ahead with the plant, it will be required to draft a regulation on nuclear safety practices in line with International Atomic Energy Agency guidelines.