The movement appeared to be gaining traction across the country with smaller protests taking place Sunday from Phuket in the south to Khon Kaen in the northeast.
Thailand’s embattled premier called Monday for a special session of parliament as protesters planned more rallies to demand his resignation, the release of jailed activists, and reforms to the monarchy.
Tens of thousands of mostly young protesters have taken to the streets in the past week in defiance of an emergency decree banning gatherings of more than four people.
Police said around 20,000 people protested across the capital Sunday, although activists and local media estimated much bigger crowds.
As they prepared to rally again Monday, Prime Minister Prayut Chan-O-Cha said parliament — currently in recess — would be recalled to discuss how to reduce tensions.
“We support opening an extraordinary session to solve this conflict,” he told reporters, warning protesters not to break the law.
“I request protesters rally peacefully. The government has already compromised to some degree,” he said.
The largely leaderless movement is calling for the resignation of Prayut — a former army chief and mastermind of a 2014 coup — as well as the re-writing of the military-drafted constitution they say rigged last year’s election in his favour.
Most controversially, protesters are also making unprecedented demands to reform the powerful and ultra-wealthy monarchy.
They want the abolition of a draconian defamation law that shields King Maha Vajiralongkorn from criticism, greater transparency of royal finances, and for the monarch to stay out of politics.
‘Protect the monarchy’
It has gained momentum since July, but sharply escalated last week after a group of protesters surrounded a royal motorcade and flashed three-fingered “democracy salutes” — borrowed from the “Hunger Games” movies — at Queen Suthida.
Two activists now face charges under a rarely used law banning “violence against the queen” and face a maximum sentence of life in prison if convicted.
Confrontations escalated further on Friday when riot police used water cannon and other strong-arm tactics, provoking widespread outrage.
Prayut warned Monday the government needed to protect the monarchy.
“This is the duty of all Thais,” he told reporters.
Apart from arrests by police, the Ministry of Digital Economy and Society said it had flagged more than 325,000 messages on social media platforms that violated the Computer Crimes Act, which critics say is used to muzzle dissent.
Police also warned local media outlets that their coverage of the protests would be examined for possible illegal content.
By midday, #SaveFreePress was the latest hashtag trending on Thai Twitter, one of several platforms being used by tech-savvy protesters to coordinate their activity.
They have also copied many tactics employed by Hong Kong protesters during months of frequently violent clashes there last year, including using improvised protective clothing in case of confrontations with riot police.