Democrat deputy leader Prinn Panitchpakdi announced his immediate resignation from the party on Thursday to fight sexual harassment allegations.
Mr Prinn said he decided to relinquish all positions, including his membership, in the best interests of the party, and so he could apply himself to refuting the accusation, which he strongly denied.
Although a deputy leader of the party, he is not a sitting member of parliament.
The issue was disclosed by lawyer Sittha Biabangkerd, who posted on his Facebook account on Wednesday that an 18-year-old woman had consulted him about alleged sexual harassed by a deputy party leader. The high-profile lawyer did not name the person accused.
Mr Sittha added on Thursday that the alleged harassment occurred at a restaurant in a Bangkok hotel on Monday. The young woman's mother accompanied her to register a complaint at Lumphini police station on Tuesday.
Mr Sittha said about 10 other women had also contacted him, making similar claims.
The lawyer said he would take up the case, alleging that the unnamed man was "a danger to society" while admitting some difficulty because the victims were "not likely to give their accounts in court".
"I will not drop this case. I will have a follow-up after I return from Phuket," he promised.
Mr Prinn on Thursday strongly denied any wrongdoing, saying he was "shocked and stunned" by the allegation.
"I insist I am innocent. I deny all allegations. They are groundless," he said. "Although what happened is a personal matter, it affects the Democrats."
Mr Prinn said he would concentrate on defending himself against the accusation and was ready to explain to authorities, instead of suing people who made the accusations against him.
"I am leaving this case to the justice process," he said.
Mr Prinn had been leading the party's economic team and was in charge of the party's campaign for both the Bangkok governor and city council elections next month, until he stepped down on Thursday.
He is the son of former deputy prime minister Supachai Panitchpakdi. Mr Supachai is also a former director-general of the World Trade Organisation and former secretary-general of the UN Conference on Trade and Development.
