Protesters on Tuesday submit a protest note to UN Education, Scientific and Cultural Organisation (Unesco) against Cambodia's bid to list Preah Vihear Temple as World Heritage Site.
The protest of about 1,000 people, in front of Unesco's Bangkok regional office started at 10am and was led by yellow shirts leaders; Chamlong Srimuang and Chaiwat Sinsuwongse. They vowed to stay on until 3pm.
The gathering has worsened the traffic congestion.
Chamlong said the protest is aimed at opposing Cambodia's listing of the temple as Unesco World Heritage Site and condemning the Unesco.
The Hindu temple near the Thai-Cambodian border was listed in July 2008 as a World Heritage Site, but Thailand has been lobbying member countries to delay Phnom Penh's management plan owing to disputes in areas adjacent to the temple.
The Thai authorities want Cambodia's management plan to be delayed until the border dispute is settled. Unsco's World Heritage panel is meeting in Brazil to discuss the list. Both countries have sent their ministers to the meeting.
The rally on Tuesday is an act in defiant of the state of emergency which has been imposed in Bangkok and other provinces.
Chamlong told reporters earlier that Thai patriots and he rallied because we are not certain that the government will succeed in opposing the temple administrative plan," Chamlong said.