Human rights lawyers have demanded the immediate release of Mr Srisuwan Janya, secretary-general of the Association of Thai Constitution Protection Organizations.
The Centre for Human Rights Lawyers said Tuesday that Mr Srisuwan, a prominent political and environmental activist, was detained by military officers when he went to the complaint centre at the Government House to petition the prime minister to order a probe into the missing plaque of the 1932 coup makers embedded at the Royal Plaza.
Prime Minister Prayut Chan-ocha has ordered the Royal Thai Office and security agencies to investigate the issue.
The lawyers' centre argued that the military had no legal basis to detain Mr Srisuwan, claiming that the detention was illegal and might constitute a breach of the international convention on political and citizenship rights that Thailand is a signatory.
The head of the lawyers'centre, Ms Yaowalak Anupan, has demanded the immediate release of Mr Srisuwan.
Source: Thai Public Broadcasting Service (Thai PBS)