"Down with feudalism, long live the people," was one of the chants at the biggest demonstration in Bangkok since Prayuth took power in a 2014 coup.
Saturday's protests moved from the campus of Thammasat University, a traditional hotbed of opposition to the military and royalist establishment, on to Sanam Luang - translated as Royal Field - outside the Grand Palace.
Organisers said there were 50,000 people present in the latest demonstration.
Police said there were at least 18,000, still enough to make it bigger than a protest last month.
Sept. 19 is the anniversary of the coup against the populist then-prime minister Thaksin Shinawatra in 2006.
Among the protesters were many of his red shirt followers, veterans of clashes a decade ago with pro-establishment yellow shirts.
So far the latest protests have been peaceful.