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.