Future Forward Party leader Thanathorn Juangroongruangkit confirmed on Wednesday he had been summonsed by police to answer a charge of sedition brought by the National Council for Peace and Order, describing it as the use of "dark power".
Mr Thanathorn wrote on his social media account on Wednesday morning that the summons was sent to his house, ordering him to report to Pathumwan police station at 10am on Saturday.
He also posted a picture of the summons issued by the Royal Thai Police Office last Friday. The document named his accuser as Col Burin Thongprapai, a legal officer of the NCPO. It did not refer to any particular act of alleged sedition.
Reuters reported the complaint against Thanathorn was filed by the army in 2015 over his involvement in helping anti-junta protesters that year, according to a police officer speaking on condition of anonymity because he was not authorised to speak to the media.
"When the dark power does not let go of Future Forward," Mr Thanathorn wrote at the beginning of his Facebook post.
He said the alleged offence was brought under Section 116 of the Criminal Code. He wondered when he committed this alleged offence. In the past year, he wrote, he had met people nationwide.
"It is clear that an old-fashioned political game not only refuses to stop after the election but also intensifies, because they are afraid of Future Forward and the unexpected victory of so many people," Mr Thanathorn wrote.
He said he would meet police as ordered and prove his innocence. He believed millions of people would support him and would not bow to the "dark power" that wanted to destroy his party.
Deputy Prime Minister Prawit Wongsuwon said on Wednesday that officials were abiding by the law and had only started an investigation to find out if the offence really occurred.
- Archive video: Thanathorn on legal charges