The arrests of 16 people on Monday who were protesting against the construction of a coal-fired power plant has drawn criticism of the military government from rights activists and environmentalists.
The planned power plant in the southern province of Songkhla will consist of two 1,000-megawatt units, and is part of a power development plan to 2036, but activists object to its expected environmental and health impact on communities in the area.
The 16 protesters were arrested on Monday as they travelled from Thepa district, the site of the plant, to the provincial capital to present a petition to Prime Minister Prayuth Chan-o-cha, who was due in the city on Tuesday for a meeting.
"This incident shows the true face of Thailand's military dictators, who have committed a long list of abuses and repressions since the May 2014 coup," Sunai Phasuk, Thailand researcher for US-based group Human Rights Watch, told Reuters.
Six protesters were injured in a scuffle with police, said anti-coal activist Supat Hasuwannakit.
"The use of force was uncalled for," Mr Supat told Reuters.
A few police officers were injured, police said, adding that the 16 had been charged with blocking traffic, assaulting authorities, and resisting arrest. Police have requested that a court detains them.
Tara Buakamsri, country director for Greenpeace Southeast Asia, said in a statement the action against the protesters reflected a "complete failure" by the government to promote a peaceful and inclusive society.
"The Thepa power plant has no legitimacy to be built," the group said.
The first unit of the power plant is due to begin operating in 2021.
Its environmental health impact assessment was completed in August and is pending approval by the National Environment Board.