Thai Prime Minister Prayut Chan-o-cha ordered security agencies on Thursday to crack down on pro-democracy protesters, days after police used tear gas and water cannon at a Bangkok rally.
The country has been rocked since July by youth-led protests demanding a new constitution, unprecedented calls to reform the untouchable monarchy, and for Prayut to resign.
Clashes outside parliament Tuesday between pro-democracy protesters and hardline royalists marked a steep rise in violence, with six people shot.
A day later, some 20,000 people massed in Bangkok's main shopping district, and protesters daubed anti-royal graffiti outside the Thai National Police headquarters.
Prayut, who seized power in a 2014 coup, issued a statement Thursday warning protesters will be hit with the full force of the law.
"The situation is still not resolved in any good direction and is likely to develop into more conflict leading to more violence," he said.
"If this is left... it may damage the nation and the most beloved institution," he added, referring to the monarchy.
He said the government and security agencies need to "intensify their practices", and enforce all sections of all laws.
This could mean more charges under the country's harsh royal defamation laws, which are routinely interpreted to include any criticism of any aspect of the monarchy -- including content posted or shared on social media.
King Maha Vajiralongkorn asked the Thai government in June to suspend using the lese majeste laws, but human rights critics say there is a host of other legislation that authorities can use to target democracy activists.
Asked if the government was giving the nod to police to pursue lese majeste charges, spokesman Anucha Burapachaisri did not rule it out.
"The protesters should respect all laws in general. We don't specify whether we would be enacting any laws specifically," he told AFP.
The king sits at the apex of Thai power, supported by the military and the kingdom's billionaire clans, and the royal family enjoys support from mostly older conservatives.
Lawmakers have this week been discussing various proposals for constitutional change, which mostly exclude any reform to the monarchy.
On Wednesday they agreed to look at two proposals for a "constitutional drafting assembly", while rejecting more far-reaching bills to revise the role of the royals and change the makeup of the senate.