Tens of thousands of pro-democracy protesters massed close to Thailand's Grand Palace on Saturday, in a huge rally calling for PM Prayut Chan-O-Cha to step down and demanding reforms to the monarchy.

The kingdom has seen near-daily gatherings of youth-led groups since mid-July calling for the resignation of Prayut, the former army chief behind the 2014 coup, and a complete overhaul of his administration.

Some are also demanding reforms to Thailand's ultra-wealthy and powerful monarchy -- a once-taboo topic in the country due to its tough royal defamation laws.

Thailand has seen near-daily gatherings from youth-led groups since mid-July demanding the resignation of Prayut, the former army chief behind the 2014 coup, and a complete overhaul of his administration
Thailand has seen near-daily gatherings from youth-led groups since mid-July demanding the resignation of Prayut, the former army chief behind the 2014 coup, and a complete overhaul of his administration AFP / Lillian SUWANRUMPHA

The burgeoning movement, partly inspired by Hong Kong's pro-democracy protests, remains largely leaderless.

But this weekend's demonstration was organised by students of Bangkok's Thammasat University -- a group that has been among the most vocal about the royal family's role in Thailand.

Human rights lawyer Anon Numpa, who has emerged as one of the key figures in the movement, took a stronger line on monarchical reform in a fiery speech late Saturday night.

The kingdom has seen near-daily gatherings of youth-led groups since mid-July calling for the resignation of Prayut, the former army chief behind the 2014 coup
The kingdom has seen near-daily gatherings of youth-led groups since mid-July calling for the resignation of Prayut, the former army chief behind the 2014 coup AFP / Lillian SUWANRUMPHA

"My knee will never kneel for dictatorship," he shouted, reiterating the protesters' calls for the royal family to stay outside of the kingdom's politics.

His speech was met with cheers from the crowd, who by nightfall had gathered at the historic Sanam Luang field in front of the Grand Palace.

"Today is one of the turning points in Thai history," protester Patipat, a 29-year-old history teacher in the crowd who declined to give a full name, told AFP.

The demonstration was organised by students of Bangkok's Thammasat University - a group that has been among the most vocal about the royal family's role in Thailand
The demonstration was organised by students of Bangkok's Thammasat University - a group that has been among the most vocal about the royal family's role in Thailand AFP / Mladen ANTONOV

Bangkok authorities estimated more than 18,000 at Saturday's demonstration, though organisers claimed a much higher turnout. AFP reporters on the ground estimated a crowd size closer to 30,000.

This would make it one of the largest gatherings the kingdom has seen since the 2014 coup.

Waving a three-fingered salute taken from the Hunger Games film trilogy, protesters huddled under a sea of umbrellas to shield from rain, their glowing smartphones bobbing in the darkness above the 12-hectare (30-acre) grassy field.

A pro-democracy protester wears a fake crown and a shirt that reads 'please realise this country belongs to the people' during the anti-government rally in Bangkok
A pro-democracy protester wears a fake crown and a shirt that reads 'please realise this country belongs to the people' during the anti-government rally in Bangkok AFP / Jack TAYLOR

"We will not stop until we have monarchy reform," said fellow activist Panusaya Sithijirawattanakul, who insisted their aim is not to abolish it -- but to "adapt it to society".

Around 10,000 uniformed and plainclothes police patrolled the area as the crowd grew through the day.

Some protesters wore fake crowns, while a group carried a cardboard model of a submarine to symbolise their displeasure over military spending.

"People can protest, but peacefully and within the law," said government spokesman Anucha Burapachaisri, declining to comment further.

Activists rallied outside Shibuya station in Tokyo in support of the Bangkok protests
Activists rallied outside Shibuya station in Tokyo in support of the Bangkok protests AFP / Philip FONG

Demonstrators vowed to continue rallying overnight at Sanam Luang, where Anon said they will place a replica at dawn of a small bronze plaque commemorating the end of royal absolutism in 1932.

The original was embedded in the ground of Bangkok's Royal Plaza for decades before it mysteriously vanished in 2017 -- which activists say is emblematic of a wider whitewashing of Thai political history.

Since 1932, Thailand has seen a cycle of violent protests and coups, with the military stepping in to stage more than a dozen putsches -- often in the name of protecting the unassailable monarchy.

The latest wave of student-led demonstrations has largely been peaceful.

But unprecedented calls for frank discussions about the royal family have sent shockwaves through the kingdom.

King Maha Vajiralongkorn sits at the apex of Thai power, buttressed by the kingdom's military and billionaire clans, and commands an estimated fortune of up to $60 billion.

The student demands include greater accounting of the palace's finances, the abolition of royal defamation laws and for the king to stay out of politics.

They also want a rewrite of the 2017 military-scripted constitution, which they say tilted last year's election in Prayut's favour, and for the government to stop "harassing" political opponents.

So far, authorities have arrested more than two dozen activists, including Anon, charging them with sedition before releasing them on bail.

The weekend demonstrations will prove a test for the pro-democracy movement, analysts say, who question if its support base will grow beyond the social media-savvy youth.

Thammasat University, where protesters gathered Saturday, has long been a hotbed of student activism.

In 1976 students at the university protesting the return of a military dictator were shot, beaten to death and lynched by state forces and royalist mobs.

Prayut has vowed authorities would use "soft measures" on the protesters "because they are children".

Demonstrators gathered in Tokyo, Stockholm, and Taipei Saturday in solidarity with the Bangkok protesters, with further rallies planned in the United States and France.