The pro-democracy protesters have been doing their best to elude the authorities, using social media to assemble followers before police have time to block them.
The government has announced plans to take legal action against Twitter and Facebook accounts announcing the protests – but fresh calls to action were still posted on Saturday.
The protesters are calling for prime minister Prayuth Chan-ocha to leave office, as well as an amendment to the constitution to make it more democratic and the reform for the nation’s monarchy.
The protests have been called in defiance of a state of emergency imposed on Thursday.
All stations of Bangkok’s elevated Skytrain mass transit system were ordered closed on Saturday afternoon in an effort to thwart protests.
A line of the underground MRT system was also shut, and police blocked off several roads. Organisers had called for their followers to meet at Skytrain stations pending further instructions.
Police had taken similar action on Friday to block demonstrators, but failed when protest organisers announced a last-minute switch of venue.
Friday’s rally was broken up by a large contingent of riot police backed by water cannon after at least 1,000 people had gathered.
It was the first time the authorities have employed such forceful tactics against the student-led protests.
Both Thursday’s and Friday’s protests were held in normally busy areas of central Bangkok.
One protester, a 26-year-old who declined to give her name, said: “The government has been pushing us to the cliff and now we have nowhere to go. We need to stand right now; if not now, then we don’t know when.”
Police took control of the rally site after about an hour, though continued to engage with some stragglers.
Flash protests were launched in solidarity at several universities around the country.
The People’s Party, the umbrella organisation for the protesters, said: “From the dispersal of protesters on the evening of October 16, we have learned that the government and military have established themselves as the enemy of the people.”
It vowed to go ahead with Saturday’s protest. Most of its top leaders have been arrested.
The call by the protesters for reform of the monarchy has significantly raised the political temperature in Thailand, angering many older conservative Thais for whom any critical discussion of the monarchy is tantamount to treason.
King Maha Vajiralongkorn and other key members of the royal family are protected by a lese majeste law that has regularly been used to silence critics who risk up to 15 years in prison if deemed to have insulted the institution.
Mr Prayuth said his declaration of a state of emergency was necessary because “certain groups of perpetrators intended to instigate an untoward incident and movement in the Bangkok area by way of various methods and via different channels, including causing obstruction to the royal motorcade”.
He was referring to an incident Wednesday that showed some members of a small crowd heckling a motorcade carrying Queen Suthida and Prince Dipangkorn as it slowly passed.
On Friday, two activists were arrested under a law covering violence against the queen for their alleged part in the incident.
They could face up to life in prison if convicted. They denied any wrongdoing.
