The army takeovers came in the context of constant violent protests and clashes between the conservative faction (known as the Yellow Shirts) and the Red Shirts, backers of ousted prime ministers Thaksin Shinawatra (in the office between 2001-06) and his sister Yingluck (2011-14).
Supporters of Prayuth, who as army chief, had led the 2014 coup, put nationalism on display by brandishing the Thai flag, while branding Thanathorn and his supporters "nation haters" for their liberal views.
Thousands of Thais joined a run in the capital Sunday in what appeared to be the biggest show of dissent against the government of Prime Minister Prayuth Chan-ocha, although he also drew a big show of support at a rival event.
Police estimated more than 12,745 runners and supporters gathered before sunrise at a park in Bangkok, wearing athletic outfits and colourful shoes, to participate in the 'Run Against Dictatorship event.
Thousands of anti-government protesters in Thailand donned sports gear in the pre-dawn darkness for the biggest political protest in the country in years..
It prompted a rival "Walk to Cheer the Uncle" event, held in another park, about 14km away, where thousands also turned out to show support for Mr Prayut.
Last year's general election was meant to restore full democracy, five years after the military staged the coup.
The government's sluggish economic performance has added to a growing sense of discontent. "The economy is worse and people are facing difficulties including freedom of expression".
The trigger for Sunday's rally were moves by courts to dissolve a popular, new progressive political party.
The Future Forward Party performed spectacularly in last year's election, coming from nowhere to win 80 seats. This is the demonstration of people's anger.
Thailand's government is headed by Mr Prayut, 65, after an election in March past year that the opposition described as having been manipulated to favour the leader's pro-army party. Many now assume the party will be found guilty and dissolved, possibly even this month.
"I believe that in order for Thailand to be able to be a democratic country again, the first step is that General Prayuth has to get out and the people here today I think we share that feeling".
"People should not use the word "dictator", she said.
Similar runs were organized in other provinces Sunday, leaving many to wonder whether Thailand is heading for another prolonged bout of street politics. The unrest led to two coups and led to more than a hundred deaths.