The demonstrators, a collection of pro-democracy groups including students, made no secret of their Valentine's Day protest plans, advertising them on Facebook.
In response the authorities put up a series of barriers to stop demonstrators accessing a plaza opposite a mall in the city's downtown Siam shopping district.
But dozens of activists nonetheless managed to gather on the streets and pedestrian walkways in the late afternoon, watched by a similar number of police officers.
Several of the most vocal protesters were seen being led away by police.
Also Read
"I am here to protest against the coup," Pat Lertkeerstikul, an office worker, told AFP.
"It's almost a year since they stole the last election. We want our democracy back," she added.
Others protesters handed out copies of Orwell's anti-authoritarian novel "1984" and flashed the three-fingered salute from the popular Hollywood franchise "The Hunger Games".
Since Army Chief-turned-Prime Minister Prayut Chan-O-Cha declared martial law and seized power, sweeping curbs have been placed on freedoms.
The military have banned political gatherings, censored the media, arrested and detained opponents and ramped up prosecutions under the country's controversial lese majeste laws.
The junta says it will hold fresh elections in early 2016 once reforms to tackle corruption and curb the power of political parties are codified in a new constitution.
Some analysts and Shinawatra supporters fear those constitutional changes are designed to ensure the family is politically neutralised in any upcoming election.