Three activists from the "Free People Thailand 58" group were taken into police custody on Sunday for staging a protest in front of a department store at Victory Monument.
Akrakrit Noonchan, 47; Worachart Pipakpimcharoen, 42; and Sarinee Boonchu, 45, were detained in front of the Centre One department store.
The group, led by Mr Akrakrit, was demanding the government hold a weekly people's opinion forum, and distributing to onlookers and reporters T-shirts featuring a bird with its beak and legs bound.
Leaflets were also distributed, explaining the group's protest was not against the government or a challenge to martial law. Members also denied they had been hired to stage their rally.
The group wants the government to launch a weekly forum to gather opinions on how to tackle high goods prices, the increasing cost of living and other issues, the leaflet said.
It said public participation would be needed if the government hoped to solve these problems, as it could not do the job alone.
It also called on the government to relax restrictions on public expressions of opinion.
While the three were waiting for more members of the group to arrive, a band of police and soldiers deployed ahead of the protest escorted them to a nearby police booth.
Mr Akrakrit, Mr Worachart and Ms Sarinee were later taken to Phayathai station for further questioning and "attitude adjustment".
They were later released after being charged with causing a public disturbance and fined 200 baht each.