Anti-government activists on Wednesday announced plans to hold an overnight protest and to rally up to 100,000 people to march on Government House next week, piling more pressure on the military-backed administration to step down.
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.
"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.