Future Forward Party leader Thanathorn Juangroongruangkit was summonsed for alleged sedition because anti-coup demonstrators fled in his van after protesting outside Pathumwan police station in Bangkok in 2015, according to the National Council for Peace and Order.
Col Burin Thongprapai, legal affairs chief of the NCPO, said on Wednesday that about 20 demonstrators had converged on Pathumwan police station in 2015.
Police and soldiers tried to arrest some them who were wanted under arrest warrants, but they escaped, he said.
The demonstrators included Rangsiman Rome. He and other demonstrators got into a van and fled the scene. Officials found out later that the van belonged to Mr Thanathorn, Col Burin said.
Mr Ransiman is now assistant secretary-general of Mr Thanathorn's Future Forward Party, which was founded only in March last year.
On that day in 2015 Mr Thanathorn was in the area and posted a message related to the incident on his Facebook account, and that led to the summons, Col Burin said.
A police source said that the incident happened on June 24, 2015, when Mr Rangsiman and other demonstrators were summonsed to acknowledge sedition charges in relation to their rally against the May 22, 2014 coup. The May 22, 2015 demonstration outside the Bangkok Art and Cultural Centre marked the first anniversary of the coup by the NCPO.
At the police station the group protested against police attempts to end their rally.
The group faced arrest warrants for alleged sedition, a violation to Section 116 of the Criminal Code. The warrants concerned their anti-coup rally outside the Bangkok Art and Cultural Centre.
Earlier on Wednesday, Mr Thanathorn commented on his summons, saying the regime was afraid of his party and was exercising "dark power".