The ruling junta has kept a tight lid on dissent since its 2014 coup by outlawing gatherings of more than five people and ramping up use of a draconian royal defamation law.
But impatience with the generals has been building in recent weeks and breaking through the surface in a series of creative and increasingly bold protests.
The agitation has been fuelled by a spate of graft scandals, including one that has ensnared the junta's No 2, plus the postponement of elections the regime had promised for November 2018.
"We want to get our democracy back," Sumittra Patprathun, a middle-aged Bangkokian in the crowd, told AFP as activists took turns railing against the military regime through a megaphone.
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Hundreds of police were deployed to monitor the gathering.
Thirty-nine people who joined a similar rally in Bangkok last month were formally charged this week with violating the junta's ban on gatherings and a public assembly law.
Nine of those activists have also been hit with the heavier offence of sedition, which carries up to seven years in prison, for organising the rally.
One of those activists, Ekachai Hongkangwan, was detained at his home on Saturday morning ahead of the rally, which the three others attended.
The junta has repeatedly reneged on promised elections since its power grab, which it says was necessary to return order after a decade of political turmoil.
Four years on, patience with military rule is dwindling as frustration grows over the regime's lack of transparency and a culture of impunity for the wealthy and well-connected.
Much of the ire has been fixed on junta No 2 Prawit Wongsuwan, who is being probed for an eye-popping collection of some two dozen luxury watches he allegedly failed to declare in his assets.