Jatuporn Prompan and Nattawut Saikuar, leading members of the United Front for Democracy against Dictatorship, were arrested by soldiers at Mahachai market in Muang district of Samut Sakhon province on Monday morning, according to media reports.
The two UDD leaders were reportedly talking to reporters at Mahachai market before going to Rajabhakti Park in Hua Hin district, Prachuap Khiri Khan province, when they were apprehended and taken away by the military.
It was not known where they were being taken.
Earlier today, Mr Nattawut posted on his Facebook page that a Toyota Camry car was seen in front of his house at Sanam Binnam in Nonthaburi province throughout the night, accompanied by two motorcycles, possibly from a rapid deployment unit.
The men inside the vehicle were plain-clothes personnel, he wrote.
Mr Nattawut continued: "No matter what, I will definitely go to Rajabhakti."
He arrived at Mahachai shortly after 10am. There, he and Mr Jatuporn held a press conference, saying they planned to visit Rajabhakti Park to pay homage to the statues of the seven past kings.
The visit was neither politically motivated nor intended to apply pressure on the government, they said.
Soldiers also arrest Jatuporn Prompan at Mahachai market.