A member of the Free Guards group reported to have been kidnapped by state security personnel was found near the BTS station in Samut Prakan's Bang Phu area on Sunday afternoon.
The Internal Security Operations Command (Isoc) denied any involvement in Mongkhol Santimethakul's disappearance.
Mr Mongkhol, 23 and nicknamed "Yel", was accompanied by lawyers yesterday when he lodged a complaint at Samut Prakan police station claiming he had been kidnapped.
He told the media he was hustled into a van by five men at 11pm on Saturday as he went to shop at a 7-Eleven store on Phraeksa Road in Samut Prakan province.
He said he could not identify the faces of his abductors as he was blindfolded. He told police the men blamed him for placing a banner with a lese majeste message in front of Bang Pu Recreation Centre early this month, a claim he denied.
He told police one of the abductors forced him to unlock his mobile phone and used his Line chat account to communicate before releasing him yesterday afternoon near the BTS station.
News of his disappearance was reported on Saturday following a post on his Facebook page which read: "Help me".
On Saturday the Free Guards group had protested near Victory Monument where activists unfurled a 112-metre-long banner protesting against the lese majeste law and the government. After Mr Mongkhol's disappearance, members of the Free Guards group had received messages from an account in Mr Mongkhol's name in a Line app group, instructing his friends to pick him up, according to Pompuen Parnpuangkaew, one of the members of the group.
Mr Pompuen said the abductors wrote that they were staff members of Isoc and made a threat to kidnap others and made other threats.
At 1pm on Sunday, Mr Pompuen said the men sent another message on Line instructing the group to pick up Mr Mongkhol at the BTS Station in Bang Pu where the activist was found safe but in a near incoherent state.