Anti-government protesters have installed a plaque declaring Thailand "belongs to the people", in a bold show of opposition to the monarchy.
The plaque was laid near Bangkok's Grand Palace in the latest challenge to Thai King Maha Vajiralongkorn.
Student-led protests calling for reform of the country's monarchy and political system have been going on since July.
The protests over the weekend were some of the largest in years, with thousands defying authorities to demand change.
Authorities say 18,000 people joined Saturday's demonstration, although others give higher figures. Many stayed to continue the protest into Sunday, before dispersing.
Their calls for royal reform are particularly sensitive in Thailand, where criticism of the monarchy is punishable by long prison sentences.
Protesters are also demanding the resignation of Prime Minister Prayuth Chan-ocha, who took power in a 2014 coup and won disputed elections last year.
On Sunday morning, student activists cemented a commemorative "People's Plaque" close to a field known as Sanam Luang, or Royal Field.
The plaque, dated 20 September, 2020, proclaims in Thai: "The people have expressed the intention that this country belongs to the people, and not the king."
Organisers said the plaque was a replacement for another marking the end of absolute monarchy in the 1930s, which went missing in 2017.
Cheers erupted as activists installed the new plaque, with protesters chanting: "Down with feudalism, long live the people."
Police did not intervene and there were no reports of violence. A spokesman for the Thai government told Reuters news agency police would not use violence against protesters.
Later on, protesters who had planned to march to Government House were blocked from doing so by hundreds of unarmed police manning crowd control barriers.
Instead, the protesters marched to hand a letter of demands for reform of the monarchy to the king's Royal Guard police.
Protest leaders declared victory after saying Royal Guard police had agreed to pass on their demands to police headquarters. There has been no comment from the police.
"Our greatest victory in the two days is to show that ordinary people like us can send a letter to royals," protest leader Parit "Penguin" Chiwarak said, telling crowds to return for another demonstration next week.
Why are there protests?
Thailand has a long history of political unrest and protest, but a new wave began in February after a court ordered a fledgling pro-democracy opposition party to dissolve.
The Future Forward Party (FFP) had proved particularly popular with young, first-time voters and garnered the third-largest share of parliamentary seats in the March 2019 election, which was won by the incumbent military leadership.
Protests were re-energised in June when prominent pro-democracy activist Wanchalearm Satsaksit went missing in Cambodia, where he had been in exile since the 2014 military coup.
His whereabouts remain unknown and protesters accuse the Thai state of orchestrating his kidnapping - something the police and government have denied. Since July there have been regular student-led street protests.
Demonstrators have demanded that the government headed by Prime Minister Chan-ocha, a former army chief who seized power in the coup, be dissolved; that the constitution be rewritten; that the authorities stop harassing critics.
What is different this time?
The demands of protesters took an unprecedented turn last month when a 10-point call for reform to the monarchy was read out at one rally.
The move sent shockwaves through a country which is taught from birth to revere and love the monarchy and fear the consequences of talking about it.
The young woman who delivered the manifesto, Panusaya Sithijirawattanakul, has said their intention "is not to destroy the monarchy but to modernise it, to adapt it to our society".
But she and her fellow activists have been accused of "chung chart" - a Thai term meaning "hatred of the nation" - and they say they are deeply fearful of the consequences of doing "the right thing" by speaking out.
What are the laws protecting the monarchy?
Each of Thailand's 19 constitutions of modern times has stated, at the top, that: "The King shall be enthroned in a position of revered worship" and that "no person shall expose the King to any sort of accusation or action".
These provisions are backed by article 112 of the criminal code, known as the lese-majeste law, which subjects anyone criticising the royal family to secret trials and long prison sentences.
The definition of what constitutes an insult to the monarchy is unclear and human rights groups say the law has often been used as a political tool to curb free speech and opposition calls for reform and change.
The law had been increasingly enforced in the years after the 2014 coup, although it has slowed since King Vajiralongkorn let it be known he no longer wanted it to be so widely used.
But observers say the government has used other legal routes, including the sedition law, to target dissent.