A march on the seat of government would mark the first time the pro-democracy movement, which started rallies in July, moved beyond speeches on stages, reports the Bangkok Post.
“We will stay overnight on university campus on Sept 19 and march the next day,” said Panusaya Sithijirawattankul, a leader of the United Front of Thammasat and Demonstration.
If more people join, the rally would move to Sanam Luang, she said during a press conference held at Thammasat University.
After a long absence, demonstrations have returned to the country’s streets to demand the removal of Prime Minister Prayut Chan-o-cha, reviving memories of over a decade of intermittent unrest that culminated in a 2014 coup.
Gen Prayut led that putsch and remained in power after a disputed election last year that he insists was conducted fairly. He has warned Thailand could be “engulfed in flames” if protests persist.
The protesters want the government to stop harassing opponents, amend the constitution and dissolve parliament.
Some demonstrators have also broken a longstanding taboo by calling for curbs on the power of the country’s monarchy.
“We will certainly talk about the reform of the monarchy on stage on the 19th,” said Parit Chiwarak, a protest leader better known as “Penguin”.
The rally coincides with the anniversary of a 2006 coup that ousted the government of Thaksin Shinawatra.
The 2014 coup removed a government led by his sister, Yingluck. Both Shinawatras were elected in landslides.
The protesters pledged a peaceful rally.
“We don’t have weapons. Don’t use tear gas on us, don’t harass the people,” Mr Parit said, adding he expected a turnout of 50,000-100,000 people.
There have been only minor confrontations so far and protest leaders who were arrested have all been released on bail.
Government spokesman Anucha Burapachaisri said the rally must be peaceful and lawful.
“The government is concerned about safety,” he said.
Mr Parit, a core member of the students’ Free Youth group, said people taking part in the rally at Thammasat University’s Tha Prachan campus on Sept 19 would camp out the night before, then march to Government House in the morning in a bid to “demand democracy”.
He announced on Wednesday that the title of the rally would be “Sept 19: Return power to civilians”.
Mr Parit said the Tha Prachan campus had been chosen because it was a “Fortress of Democracy” and had a long history of being linked with pro-democracy movements.
Mr Parit said that the group also had a Plan B, so that if the campus became overcrowded, the rally would be moved to the nearby Sanam Luang grounds, a gesture he described as “occupying Sanam Luang and returning it to public”.
“I’m sure we will be able to use Sanam Luang, even though it is prohibited by law from being used [for a political gathering] because it is a part of a palace,” he said.
“Yet, civilians own the land of this country too,” he added.
He also said the rally intended to turn Ratchadamnoen Avenue into a space for art and political activities relevant to the group’s movement.
The rally is due to begin at 2pm on Sept 19 and the march to Government House would begin at 8am the following day, said Mr Parit.
Gen Thirawat Bunyawat, army chief-of-staff and secretary-general of the Internal Security Operations Command (Isoc), said the authorities had closely monitored the organisers’ plans and prepared measures to handle potential confrontations. Soldiers assigned to monitor the protests have been ordered to do so peacefully.
Thammasat University, meanwhile, has announced that political gatherings held on its campuses had to follow three pre-conditions: they needed to be legal, pre-approved and organised by certified student bodies.