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.

Some of them shouted slogans such as "Prayuth, get out!" or "Long live democracy!" while running the 2.6-km course amid a sprinkling of security forces. Many also gave a three-finger salute of resistance to authority.

"I want things to be better," said Bangkok resident Waraporn Waralak, 45, after completing her run. "I want Prayuth to get out."

Thailand's government is headed by Prayuth, 65, after an election in March 2019 that the opposition described as having been manipulated to favour the leader's pro-army party.

"This is the biggest gathering since the coup," said Anusorn Unno, dean of the sociology and anthropology faculty at Bangkok's Thammasat University, adding that the harmless nature of the activity had spurred greater participation than usual.

An excited crowd watched as organisers auctioned bibs bearing numbers significant in Thai politics, such as 2475, the Buddhist calendar year of the revolution that ended absolute monarchy in 1932.

To loud cheers, a bib numbered 0044, in reference to the former ruling junta's Article 44 that gave Prayuth absolute executive power, was ripped in two on stage.

The event was held in line with people's rights and no laws were broken, a government spokeswoman told Reuters.

The run followed a rally last month staged by Thanathorn Juangroongruangkit, the 41-year-old leader of the progressive Future Forward Party, who has emerged as the most outspoken opponent of Thailand's new civilian government.

Thanathorn was charged with breaching a law on public assembly over the December rally, and Future Forward faces dissolution this month by the constitutional court.

Australian Associated Press