The government decided on Friday to proceed with a planned 800-megawatt coal-fired power plant in the southern province of Krabi, overriding vigorous and widespread objections.
The project's opponents vowed to expand their protests after approval was announced, and tried unsuccessfully to enter Government House.
The Electricity Generating Authority of Thailand plans to build the plant in Nua Khlong district of Krabi.
Prime Minister Prayut Chan-o-cha said the Energy Policy and Planning Committee he chaired resolved to go ahead with the power plant. Authorities had already shelved it for two years to explain its benefits to the public. Now it was time to implement it as the South had need of a secure power supply, he said.
"I ask opponents not to create conflict," Gen Prayuth said.
"The South consumes more power than other regions, but has fewer power plants than other regions. More plants will be developed, with the principle of cost effectiveness, safety and benefits to local people.
"The South has experienced blackouts. The longest was three hours."
Energy Minister Anantaporn Kanjanarat said the coal-fuelled power plant would use clean and safe technology.
The South was depending on 400-500MW of electricity generated from other regions. Without a new electricity generating plant, power-saving measures would be necessary there.
Twarath Sutabutr, director-general of the Energy Policy and Planning Office, said the South's demand for electricity had risen by 4.7% in the past decade. Provinces facing the Andaman Sea had a power problem because local plants could not meet the local demand. He referred to Krabi, Phangnga, Phuket and Trang provinces.
The new power plant in Krabi would meet the demand, he said.
The decision to go head with the project upset about 200 opponents who rallied outside Government House awaiting the decision. They vowed to continue their rally there until the government changes the decision.
Representatives of protesters in the lower South said they would stage bigger rallies in the South and in Bangkok. There was no truly clean technology for coal-fired power plants. Pollution from the plant would affect local tourism and people's livelihoods.
Protesters in Bangkok tried to walk into Government House, but police stopped them.
Video by Thiti Wannamontha