The arrests came a day after the National Energy Policy Committee (NEPC), chaired by Prime Minister Prayut Chan-o-cha, gave the nod to the construction of the 800-megawatt plant to be sited in Nua Khlong district.
A brief scuffle broke out between protesters and police on Friday afternoon following the approval.
Three key coordinators of Save the Andaman from Coal were arrested at the Office of the Civil Service Commission, opposite Government House, at 9.30am on Saturday.
They were Prasitthichai Nunual, ML Rungkhun Kitiyakara and Akkaradet Chakchinda.
Two other leaders -- Banjong Nasae, president of Thai Sea Watch Association, and Thatpong Kaedum, a local community rights activist -- were taken away in the afternoon. They were detained by army military police and officers at the 11th Military Circle in Bangkok for "interrogation".
Police put 12 other activists into a van as a tussle broke out, but the vehicle managed to leave the site.
Pol Lt Gen Surachet Hakpan, chief of the Patrol and Special Operation Division, said police discharged the 12 after filing their records and adjusting their attitudes, saying that they were freed without charge.
Kwankanok Kasirawat, a member of Save the Andaman from Coal whose husband was detained by the police, said she would defy police orders to leave the area until she had met with her husband and the others.
She said there needed to be more people meeting outside Government House to show their opposition to the government's decision to go ahead with the Krabi plant.
About 100 Krabi residents were staying near to Government House and the number is expected to swell considerably after news that representatives from Save the Andaman from Coal's 51 allies are going to join the group in Bangkok.
An activist who declined to be named said the detention of the five key leaders had reduced the group's ability to move around and so the next steps must be considered carefully.
Col Winthai Suvaree, spokesman of the National Council for Peace and Order, said the leaders were taken for talks to find a solution to the issue. Police have not yet pressed any charges against them, he said.
Government spokesman Sansern Kaewkamnerd said the arrests were made because the protesters failed to disperse or move to a designated protest area set aside by authorities, who were trying to negotiate with the group.
The demonstrators had used force to pressure the officials and showed their intention to camp out at Government House, as well as incite others to join in, he said. "The 2015 Public Assembly Act has clear guidelines that demonstrators must comply with, and Government House is a restricted area [to demonstrations]," Lt Gen Sansern said.
He said the Civil Court on Monday will consider a petition brought by police to end the demonstration.
The petition was lodged by Pol Lt Col Supak Wongsawat, deputy superintendent of Dusit district police, on Friday.
Lt Gen Sansern called on the public to look at the whole picture in respect of the country and the future of the people in the South rather than focus on the protest.
He insisted there were still a lot of people supporting the coal-fired power plant which is needed to meet rising electricity demand and prevent power outages in the South.
He noted coal transporters would not obstruct the navigation routes of fishing trawlers or affect tourism in the area, while modern equipment will installed at the plant to ward off pollution.
Responding to the calls for the use of palm oil to produce electricity instead of coal, Lt Gen Sansern said the cost of palm oil-based power generation is as much as B8.42 per unit, compared with B3.78 per unit of fuel oil which is currently used to produce electricity at the Krabi power plant.
As a palm oil-based power plant could have an annual capacity of 2,400 million units of electricity, the cost of power would rise by B11 billion a year.
Meanwhile, the Electricity Generating Authority of Thailand (Egat) has vowed to press ahead with the coal-fired power plant in Krabi following the NEPC's approval of the project.
Egat deputy governor Saharath Boonpotipukdee said the agency would strictly comply with the laws in connection with the area management, including steps to foster public participation as well as enhance public input and acceptance before the project goes ahead.
The environmental health impact assessment (EHIA) of the project is now being considered by an expert committee and the National Environment Board, said Mr Saharath, who is also the Egat spokesman.
If it is approved, the project would be presented at the cabinet for approval.
He insisted the project would strictly adhere to the guidelines outlined by the tripartite committee on the coal-fired power plant project in Krabi.
"Egat affirmed the coal-fired power plant in Krabi will be environmentally friendly and it will take care of quality of life of the communities surrounding the plant," Mr Saharath said.
Meanwhile, an Egat source said the agency has made the Krabi coal power plant its top priority. If the project is not allowed to go ahead, it will make it impossible for any other coal power plant.
National human rights commissioner Angkana Neelapaijit visited the protesters and said that the human rights office is considering making a statement about reminding the government it needs to understand people's rights and the freedom to hold a peaceful protest.
She said the government should wait for the court's verdict on whether the protest is legal or not before making an arrest.
Civil networks have stepped up calls for the government to scrap the coal-fired power plant and release the detained activists.
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