Thai Prime Minister Prayuth Chan-ocha said student protesters went too far after some issued a 10-point demand to reform the monarchy, which is considered sacred in the country's conservative culture.
Student protesters carrying anti-government banners regrouped at the University of the Thai Chamber of Commerce in Bangkok calling on Tuesday night for reforms while demanding that Prayuth resign.
The students carried placards that read: "If we burn, you burn with us." Others held signs showing a photo of a Thai activist Siam Thirawut, who went missing in 2019.
The night before, about 3,000 to 4,000 protesters chanted "long live democracy" at Thammasat University on the outskirts of the Thai capital.
Speeches were delivered calling for the resignation of Prayuth, who first took power in a 2014 coup, and an end to military domination of politics.
But protesters from the Thammasat University Pro-Democracy Group also issued a 10-point call for royal reform, becoming at least the third student group to break a decades-long taboo on questioning the monarchy's role and powers.
Thailand has strict "lese-majeste" laws against insulting or defaming the king, punishable by up to 15 years in prison.
Palace officials declined to comment on the student protests or any criticism of the monarchy.
Prayuth, a former chief of the armed forces, told reporters he watched the protests with concern.
"There are a lot of people in trouble waiting for their problems to get fixed, not just the young people. So is doing all of this appropriate?"
"It really went too far," Prayuth said, without directly commenting on demands for royal reform.
In June, he warned protesters against involving the monarchy in their demonstrations but said King Maha Vajiralongkorn, who took the throne after the death of his father in 2016, had asked him not to arrest anyone under the "lese majeste" laws.
The monarchy remains a sensitive subject in Thai society and Monday's protest prompted a public statement by Thammasat University apologising for the event.
It said while the university was supportive of freedom of expression, it did not condone "some references on the monarchy that impact people's feelings", adding legal action would be taken.
The students' new demands included a reversal of a 2019 order that transferred two army units to the king's personal command, and a 2017 law that gave him full control of the crown's extensive property holdings.
It also urged the king to refrain from endorsing any military takeover in the future.
There have been reports the monarchy asked several media companies to remove any mention of the demands listed by the students.