Wielding iron bars and carrying knives and guns, the men attacked residents of Ban Na Nong Bong in Tambon Khao Luang, Wang Saphung district. Some victims say they were detained illegally until early yesterday.
Pornthip Hongchai, a prominent member of the Khon Rak Ban Kerd Group, said as many as 41 people were injured and one remained in hospital.
Wang Saphung Police Station superintendent Colonel Somchai Srikhamdaeng vowed to investigate the residents' complaints.
Several victims believe the armed strangers were linked to Thung Kham Co Ltd, which operates a gold mine in their neighbourhood. While the men were there, several trucks left the company's compound, reportedly carrying a huge amount of copper.
Locals suspect that some valuable minerals were brought in from elsewhere and smelted in a way that endangered their health and the environment.
Public health authorities warned locals not to use rainwater and water in the area.
Since last year, residents of Ban Na Nong Bong have taken turns guarding the road in and out of the compound to prevent any delivery of valuable minerals in the hope their environment and health will get better if the company finally decides to leave.
Col Somchai promised to answer the residents' demand that the culprits be arrested quickly, to stop the illegal transportation of minerals and launch a disciplinary probe into police who failed to stop the attack.
The police chief gave the assurances only after many members of the Khon Rak Ban Kerd Group rallied in front of the police station and blockaded a nearby intersection yesterday.
"Police and relevant officials should step in to ensure the safety of people," protest leader Samai Pakmee demanded.
Wirorn Rujiyaiwat lamented that locals were frightened as the strangers attacked them and took away their valuables such as cameras, gold necklaces and cell phones.
"It's as if we were in the war zone," she said.
Yon Khunna, who was watching out for the village on Thursday night, said the men tied his hands and beat him up for hours.
"I was released just at 4.30am," he said.
Somchai said police rushed to the scene when the incident was reported on Thursday night but locals prevented police from enter the village.
"They were afraid our presence would complicate the situation because things happened in the darkness," Somchai said.
He said police finally inspected the village and gathered evidence during the daytime.
A brick and concrete wall erected by locals to block the company's operation has been brought down.
The Network of Lecturers and Students for Society and the Environment issued a statement condemning the attack.
"This is a grave violation against human rights and community rights," the group said.