Protest leader Parit "Penguin" Chiwarak declared victory and told the crowd that the next protest demanding a new constitution and reforms to curb the powers of the monarchy would be on Thursday outside parliament.
Openly challenging the monarchy of Thai King Maha Vajiralongkorn, thousands of protesters march in Bangkok on Sunday to present demands that include a call for reforms to curb his powers.
Protesters have grown ever bolder during two months of demonstrations against Thailand's palace and military-dominated establishment, breaking a longstanding taboo on criticising the monarchy, which is illegal under lese majeste laws.
The royal palace was not immediately available for comment. The king, who spends much of his time in Europe, is not in Thailand now.
The marchers were blocked by hundreds of unarmed police manning crowd control barriers.
'Our greatest victory'
Protest leaders declared victory after handing police a letter detailing their demands. Phakphong Phongphetra, head of the Metropolitan Police Bureau, said on a video broadcast from the scene that the letter would be handed to police headquarters to decide how to proceed.
"Our greatest victory in the two days is showing that ordinary people like us can send a letter to royals," Parit "Penguin" Chiwarak told the crowd before it dispersed.
At the biggest demonstration in years, tens of thousands of protesters on Saturday cheered calls for reform of the monarchy as well as for the removal of Prime Minister Prayuth Chan-ocha, a former junta leader, and a new constitution and elections.
Shortly after sunrise on Sunday, protesters cemented a plaque near the Grand Palace in Bangkok in the area known as Sanam Luang, or Royal Field.
It reads "at this place the people have expressed their will: that this country belongs to the people and is not the property of the monarch as they have deceived us."
Government spokesman Anucha Burapachaisri said police would not use violence against protesters and it was up to the police to determine and prosecute any illegal speech.
Bangkok authorities would need to determine whether the plaque is illegal and if it is it would need to be removed, Bangkok's deputy police chief Piya Tawichai told reporters.
Far from all Thais support the new plaque, which resembles one that had commemorated the end of absolute monarchy in 1932 and which was removed from outside a royal palace in 2017, after Vajiralongkorn took the throne.
Prominent right-wing politician Warong Dechgitvigrom said the actions of the protesters were inappropriate and that the king was above politics.
"It didn’t achieve anything," he told Reuters. "These actions are symbolically against the king, but the king is not an opponent."
Thai authorities have said criticising t he monarchy is unacceptable in a country where the king is constitutionally "enthroned in a position of revered worship".
Protests that began on university campuses have drawn increasing numbers of older people. That includes red shirt followers of ousted populist prime minister Thaksin Shinawatra who had clashed for years with pro-establishment yellow shirts before Prayuth seized power in 2014.
"The new generation is achieving what their parents and grandparents didn't dare. I'm very proud of that," said Somporn Outsa, 50, a red shirt veteran. "We still respect the monarchy, but it should be under the constitution."
Protesters say the constitution gives the king too much power and that it was engineered to allow Prayuth to keep power after elections last year. He says that vote was fair.
The next protest is scheduled for Thursday. Protest leaders called on Thais to take October 14 off work to show their support for change.
“Radical change is hard in Thailand, but the movement has at least kept the momentum going," said Titipol Phakdeewanich, dean of political science at Ubon Ratchathani University.