The Pheu Thai Party wrote to the United Nations High Commissioner for Human Rights on Thursday to complain about the detention of one of its key figures, Watana Muangsook, and accused the government of serious rights violations.
Acting Pheu Thai leader Viroj Pao-In wrote to commissioner Zeid Ra'ad Al Hussein to draw his attention to "the growing crisis in Thailand, which stems from the continuing violation of human rights and undermining of the rule of law".
He recounted how soldiers surrounded and entered the home of Mr Watana, "and, ultimately, unlawfully taking him and holding him in custody at an undisclosed location."
"Although he has now been released, he will be faced further with a criminal lawsuit against the Computer Related Crime Act B.E. 2007 for simply expressing his comment requesting NCPO to stop a violation of human rights," the letter wrote, referring to the National Council for Peace and Order.
"These acts are disastrously in violation of human rights and the rule of law."
A few Pheu Thai members also raised the matter with a representative of the office of the high commissioner in Bangkok on Thursday.