Thousands of protesters marched in Bangkok on Sunday (September 20) in an open challenge to Thai King Maha Vajiralongkorn.
In a country where lese majeste laws make criticizing the monarchy illegal - this show of defiance is tearing up a longstanding taboo.
But protesters, emboldened over two months of demonstrations, say the king has too much power and want reform.
Sunday's action culminated in a letter of demands being handed to police.
[Protest leader Panusaya "Rung" Sithijirawatanakul, saying:] "We have handed over the open letter to the King to the police. In two days, we have shown our strengths."
Police said the letter would be handed to headquarters where they would decide what to do with it.
The Royal Palace was not immediately available for comment and the king, who spends much of his time in Europe, is not currently in Thailand.
Thai authorities say criticizing the monarchy is unacceptable in a country where the king is constitutionally "enthroned in a position of revered worship".
But protesters want that constitution reformed, saying it was also engineered to keep Prime Minister Prayuth Chan-ocha in power after elections last year.
They're calling for the former junta leader's removal, though he says the vote was fair.
Protest leader Parit Chiwarak said Sunday's demonstration has shown that the military-dominated establishment and monarchy are equal with the people, saying "we have the same red blood - we've proven that nobody bleeds blue blood.
The next protest is scheduled for Thursday (September 24) and protest leaders have called on Thais to take October 14 off work to show their support for change.