About 600 police officers were deployed to keep the main protest at a shopping complex in Bangkok under control, the Bangkok Post reported.
Police Maj. Gen. Thitirat Nonghanpitak said police were there to prevent clashes between those protesting over their dissatisfaction with Prime Minister Yingluck Shinawatra's government and pro-government counter-protesters.
The V for Thailand website said the protest was to start at the shopping center, then move to the Bangkok Metropolitan Administration's arts and cultural center where protesters were to sing the national anthem before dispersing.
Many protesters wore Guy Fawkes masks.
There was no report of violence or injuries.