The protesters included a Thai helicopter pilot group wanting the NCPO to act against foreign pilots working for nominee firms, despite the profession being reserved by law for Thais.
Another group of Bangkok city workers said they were in heavy debt due to the city's retrospective order to cut salaries; and a third were dispossessed villagers from Buri Ram.
The helicopter pilots, led by labour union president Kiattipoom Saengthi, claimed nominee companies in the past decade had hired foreign pilots to fly in Thailand, accounting for 50 per cent of jobs, especially in oil-drilling bases off the Gulf.
They wanted the NCPO, which is now regulating migrant workers, to check on foreign pilots.
The Bangkok Metropolitan Administration employees, led by Choti Khiewchan, said they were affected by the BMA's December 2013 order reducing salaries of 500 city workers with retrospective effect from April 1, 2010.
The workers, whose pay cheques have been shrinking since January, were told on April 10 to sign their approval within 15 days for retrospective deduction from their current salaries or face lawsuits, Choti said.
The group wanted the BMA to explain the three-year retrospective salary cut, which was unfair because none of the workers had done any wrong, he said. The city tried to deduct Bt30 million from the workers' current salary total - a six-digit figure in baht per worker, he added.
"We get smaller pay cheques and are forced to pay a "debt", while the BMA did not probe or punish the persons who had caused this damage.
The BMA holds the workers accountable for a wrongdoing they didn't do. Low-ranking workers have no bargaining power and have families to take care of; some have less than Bt100 left after all cuts," Choti said.
If the BMA persisted, the group would file a lawsuit with the Administrative Court for a temporary injunction, he added.
A group of villagers from tambon Lam Nang Rong in Buri Ram's Noen Daeng district sought the NCPO's help in dealing with the Forestry Department.
They said department officials had driven them out of a eucalyptus plantation whose concession to a private sector had expired. The villagers had settled in a 500-family community there since 2007.