Pro-democracy protesters took to the streets of Bangkok for what organisers say will be the largest demonstration so far.
The protesters are calling for Prime Minister 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 Thailand due to its tough royal defamation laws.
Pro-democracy protest leader Panupong Jadnok (back left) waves flags on a truck outside Thammasat University during a protest in Bangkok. Source: AAP
The burgeoning movement, partly inspired by Hong Kong's pro-democracy protests, remains largely leaderless.
But the weekend's demonstration is 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.
At noon a crowd of hundreds forced open the locked gates of the campus, chanting "Down with dictatorship, long live democracy!" and "Prayut get out!"
"We have to support the students," Rianchai, 55, told AFP.
"We only want equality."
Prominent student activist Panusaya Sithijirawattanakul said Friday reforming the monarchy was at the top of the agenda, asserting that they hoped "to adapt it to our society".
A pro-democracy protester dressed in a zombie nun costume performs the three-finger salute at Thammasat University during a protest in Bangkok. Source: AAP
Wearing ponchos, face masks and carrying umbrellas to shield themselves from rain, demonstrators held up three-finger salutes, made popular by the Hunger Games film trilogy.
Protesters are expected to move on to the historic Sanam Luang field in front of the royal palace, where they plan to spend the night before marching to the Government House on Sunday - a move authorities have warned against.
The show of force is expected to be the largest since the 2014 coup - student activists are hoping for a turnout of more than 50,000.
Some 10,000 uniformed and plainclothes police patrolled the area as the crowd grew, setting up tents selling t-shirts, flags and beer along a main road outside the university.
'No future' for children?
A cycle of violent protests and coups has long plagued Thailand, with the arch-royalist military stepping in to stage more than a dozen putsches since the end of royal absolutism in 1932.
The latest wave of student-led demonstrations has largely been peaceful.
But unprecedented calls from some protesters for frank discussions about the monarchy 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 a fortune estimated to be worth up to $60 billion.
The student demands include greater accounting of the palace's finances, the abolition of royal defamation laws and a call for the king to remain outside 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, charging them with sedition before releasing them on bail.
The weekend demonstrations will prove a test for the pro-democracy movement, analysts say.
September 19 is also the 14th anniversary of a coup that ousted then-premier Thaksin Shinawatra from power - which the self-exiled billionaire took as an opportunity to weigh in on Thailand's current state.
"The children do not see any future... as Thailand is now out of date," Thaksin said in a statement released Saturday, which did not explicitly voice support for the movement.
Prayut has vowed authorities would use "soft measures" on the protesters "because they are children".
The top-trending hashtag on Thai Twitter on Saturday was "Sept 19, we take back the power of the people".
Demonstrators gathered in front of Tokyo's famed Shibuya station Saturday in solidarity with the Bangkok protesters, with further rallies planned in a dozen countries, including the United States and Germany.
