The runners massed before dawn in a Bangkok park for the event to take a stand against the government, which is led by former junta leader Prayut Chan-O-Cha and backed by a parliament stacked with MPs loyal to the military.
The run was led by Thanathorn Juangroongruangkit, the charismatic billionaire leader of a stridently anti-military political party which is facing the threat of dissolution.
Thanathorn has been stripped of his MP status, and is facing a battery of legal charges.
"You can feel the anger of the people and their disappointment over the government," Thanathorn told AFP before the run.
"I think this is the first step to general change in Thailand."
Before the start, the runners shouted pro-democracy slogans and chanted "Get out, Prayut".
They also wielded the three-finger salute made famous by the blockbuster Hunger Games films as a symbol of freedom from authoritarian rule.
"I want a government that takes care of the people and spends money on our well-being and the environment instead of buying tanks and submarines," said runner Gig, dressed as a tank for the run.
Prayut held onto power after elections last year, with support from an army-appointed senate.
He now holds a slim parliamentary majority and faces a public increasingly vocal in its discontent with the sluggish economy and rule by elderly former generals.
A smaller, rival "Walk to Support Uncle" rally took place in another park at the same time, drawing thousands of mainly elderly supporters of Prayut, whose nickname is "Uncle Tu".
"We love our country, we love a government which can provide security to our country," said Vasuchart, 68.
Thailand remains bitterly divided.
The contrasting demographics of the Sunday events reflected some of those splits in age, class and politics, with many older people leaning towards the army-aligned establishment and younger participants favouring Thanathorn's change narrative.
The anti-government run comes less than 10 days before Thanathorn's Future Forward Party faces possible dissolution, accused of attempting to overthrow Thailand's constitutional monarchy.
Future Forward stunned Thailand's establishment by becoming the third-largest political party after an election last March.
Despite his legal woes, Thanathorn retains a rock star-like appeal among his supporters.
bur-apj/qan