Future Forward Party (FFP) leader Thanathorn Juangroongruangkit is facing a charge of sedition in connection with his alleged involvement in helping anti-coup demonstrators escape arrest in 2015.
The complaint against him was lodged by the National Council for Peace and Order (NCPO).
A summons issued by the police, was sent to Mr Thanatorn's residence, ordering him to report to Pathumwan police station at 10am on Saturday.
According to the charge, Mr Thanathorn is accused of sedition, inciting unrest and causing a public disturbance under Section 116 of the Criminal Code, as well as aiding suspects involved in illegal activities in breach of Section 189 of the same law.
Mr Thanathorn, who leads the political party that won at least 80 seats according to the preliminary results of the latest election, said a "dark power" is behind the sedition charge.
"When the dark power refuses to loosen its grip on the Future Forward [Party], it becomes clear that the old-fashioned political game hasn't stopped," he wrote.
"It has instead intensified because they are afraid of our unexpected victory and the voice of the people."
Col Burin Thongprapai, an NCPO legal officer, yesterday confirmed the NCPO filed the complaint against Mr Thanathorn in connection with his alleged involvement in aiding anti-coup protesters in 2015.
Mr Thanathorn was summonsed because anti-coup demonstrators fled in a van, which the NCPO alleged to be linked to him after they protested outside Bangkok's Pathumwan police station that year, Col Burin said.
Col Burin said that about 20 demonstrators had converged on Pathumwan police station in 2015. "When police and soldiers, armed with arrest warrants, tried to arrest them, they managed to escape," he said.
The demonstrators included Rangsiman Rome, who is now the assistant secretary-general of the FFP. "He and the other demonstrators got into the van and fled the scene," Col Burin said.
On that day, Mr Thanathorn was in the area and posted a message related to the incident on his Facebook page, which became the basis for the summons, Col Burin said.
A police source said the incident happened on June 24, 2015, when Mr Rangsiman and six other demonstrators were summonsed to acknowledge sedition charges at Pathumwan police station.
The Bangkok Military Court had issued warrants for their arrest, following an anti-coup rally they staged outside the Bangkok Art and Cultural Centre on May 22, 2015 to mark the first anniversary of the NCPO coup.
In addition to the sedition charge, Mr Thanathorn is also facing charges related to the shares he owns in a media company, and another involving a blind trust.
On March 25, Mr Srisuwan, secretary-general of the Association for the Protection of the Constitution petitioned the Election Commission (EC) to investigate whether Mr Thanathorn was qualified to be an MP.
Mr Srisuwan claimed Mr Thanathorn applied to be an MP candidate while still holding shares in V-Luck Media Co, which runs several media outlets.
"This may constitute a breach of Section 98 of the constitution, which bars owners or shareholders of media and/or publishing companies from applying to be MPs," he said.
An EC source said yesterday that the poll agency has accepted the petition and set up an inquiry panel to look into the case.
The EC has also accepted another complaint by Mr Sriwusan to probe Mr Thanathorn's claim that he was the first to transfer his assets to a blind trust, the source said.
Yesterday, #savethanathorn became the top trending topic on Twitter in Thailand after news emerged that he was summonsed to answer the sedition charge.