Korn Chatukavanij, deputy leader of Democrat Party, led supporters to submit a letter to the US embassy on Witthayu Road declaring the coalition led by Prime Minister Yingluck Shinawatra has no legimitacy to govern.
Anti-government protesters gather at BTS Asok on Friday before noon.
Mr Korn, on Friday 7am, posted on his Facebook page a request for the gathering of supporters at BTS Asok station at 11.30am, and to later march to the US embassy. He said his group would be joined by the protesters gathered on Phetchaburi Road and march to the headquarters of the Pheu Thai Party situated on the same road.
Mr Korn was criticised by protest leader Suthep Thaugsuban, a former Democrat MP, on Thursday night for publicly disagreeing with the occupying of the Finance Ministry and other state buildings.
Members of the Business Club for Democracy and alumni and current students of many educational institutes were also gathered at Asok intersection to blow whistles in defiance of the government and join the march to the US embassy.
Democrat leader Abhisit Vejjajiva joined in the whistle blowing at BTS Asok station at 12.34pm. He said he intended to join the protest to return righteousness to the country. He insisted this protest will not end until the people triumph.
He then accompanied the protesters to the US embassy, where he delivered the protesters' letter stating the Yingluck coalition lacks the necessary legitimacy to govern the country.
Mr Korn made a speech in English from a temporary stage about 2pm, and the protesters dispersed about 2.20pm
Mr Abhisit on Thursday announced the opposition party's commitment to the campaign to remove the "Thaksin regime" and support for the law, the constitution and the courts.
Video by Apichit Jinakul