Police are hunting the people who raised a big banner reading "Reform the justice system" at several inner Bangkok locations.
The Royal Thai Police Operations Centre reported on Wednesday morning that Facebook user "Khai Maew Cheese" posted photos of the banner taken at various spots in the small hours of Wednesday.
The locations included the Giant Swing, the Democracy Monument, the Bangkok South Criminal Court, Sanam Luang, Victory Monument, the Police General Hospital and on the nameboard outside the adjacent police head office.
Securiy camera recordings showed the protesters travelling on a silver, four-door pickup truck.
The protesters were being hunted down, the operations centre said. It did not say what alleged offences they committed.