Army chief Prayuth Chan-ocha has filed a complaint with Lumpini police against Robert Amsterdam, a lawyer for the United Front for Democracy against Dictatorship (UDD) and Thaksin Shinawatra, and an unidentified woman for defamation.
He assigned Lt-Col Sayan Khunkhajee, an officer of the Judge Advocate General's Department, to file the complaint on his behalf.
According to the complaint, Mr Amsterdam went on stage to make a speech in English at the Ratchaprasong intersection on May 19 this year saying that the army bought weapons from the United States to use violence against the Thai people. He also said the US had sent military advisers to train the Thai army's snipers.
Mr Amsterdam's speech was translated into Thai by an unidentified Thai woman.
Pol Maj-Gen Anuchai Lekbamrung, deputy Metropolitan Police chief, said the Metropolitan Police Division 5 had set up a team of investigators to handle the issue and to ensure justice to both sides.