Defying heavy rain and a big police presence, around 400 protesters massed outside the US embassy, which was shut in advance of the rally, an AFP reporter said.
The planned rally was part of a wave of often violent protest against a low-budget trailer for a movie entitled "Innocence of Muslims", believed to have been produced by a small group of extremist Christians in the United States.
Holding banners in Thai, Arabic and English and flags the crowd shouted "Down with America" and called for the director of the incendiary film to be punished.
Organisers, who did not want to identify themselves as part of a specific group, were reluctant to allow journalists to speak to the protestors, shepherding them to a spokesman for questions.
"Everything that they have done is not right, it's very wrong," spokesman Niyaz -- who only gave one name -- told AFP, referring to the film.
"We are not talking about the American population, we are talking about the American government who support the film and the one that say this is a freedom of speech... freedom of speech should not harm anyone," he added.
On Monday a statement from the embassy said there was no specific threat to US citizens in Thailand, but urged Americans to "exercise caution".
Protests have been staged since last Tuesday in at least 20 countries, with more than 30 people killed and dozens wounded in violence linked to the film.
Buddhist-majority Thailand has about 1.2 million Muslims -- about 4 percent of the population -- according to the latest available data, many living in the country's southern states near the Malaysian border.