Prime Minister Prayut Chan-O-Cha said Thailand needs to bring "illegal protests" under control on Monday during a special parliamentary session called to address months of pro-democracy rallies.
Prayut, the former military chief who staged the 2014 coup, has faced increasing pressure in recent weeks from tens of thousands of protesters demanding he step down.
The student-led rallies are also calling for a rewrite of the military-scripted constitution and an end to alleged government harassment of political opponents, as well as reform of the once-unassailable monarchy.
"Though the people have the freedom to protest based on the constitution, authorities need to control the illegal protests," said Prayut, who recalled parliament from recess last week.
"We do not want to see clashes or riots in the country," he said, accusing some protesters of "inappropriate actions".
While Prayut acknowledged the protesters' demands in his opening speech, the two-day parliamentary session has not listed them on its agenda.
However, included on the list of talking points is an incident this month when protesters flashed three-finger salutes -- a symbol of their movement -- at Queen Suthida's motorcade.
Such an overt challenge is unprecedented in Thailand, where the royal family is protected under harsh anti-defamation laws and criticism of them is taboo.
Multiple opposition parties used the parliamentary session to call for Prayut's resignation, with Pheu Thai secretary Prasert Jantararuangthong saying it would "solve all problems".
"You've been in power for six-and-a-half years -- five years under your coup and one-and-a-half years when you gained benefit from an undemocratic constitution," Prasert said.
"I call for General Prayut to resign as premier, which would be a solution to solve all problems," he added.
- 'Protect the monarchy' -
Some in the leaderless youth-led movement are controversially demanding royal reform -- including the abolition of the lese majeste law, a clear accounting of the palace's finances, and for King Maha Vajiralongkorn to stay out of politics.
As the parliament session continued through the evening, thousands of protesters started marching from a major intersection to the German Embassy, in defiance of the monarch who spends long periods of time in Germany.
Protest organisers read out a statement in front of the embassy calling for Germany to "investigate" if the king has made decisions regarding Thailand from a foreign country -- which the German government has said was unacceptable.
Heading up the front of the crowd were hard-hatted protesters, linking arms in a show of solidarity as they went down Rama IV road, a massive traffic thoroughfare which had been closed hours earlier.
Dozens of police in riot gear stood guard in front of the embassy, while protesters held up signs that said "Reform the Monarchy" and held up the three-finger salute.
By 9pm (1400 GMT), the protest was called off, with organisers promising another demonstration soon.
German Foreign Minister Heiko Maas in Berlin said they were monitoring the kingdom's developments, adding that the German government is reviewing the actions of the king.
"If there are things that we think are illegal, there will be immediate consequences," he said in a press conference.
After the royal motorcade incident on October 14, the premier imposed emergency measures banning gatherings of more than four, while authorities arrested scores of activists.
Three activists were also charged under a rarely used law banning "violence against the Queen", which carries a maximum sentence of life in prison.
Earlier in the day, dozens of supporters in yellow shirts -- the royal colour -- gathered outside the German embassy, waving the Thai flag.
"We ask German government to listen to information from every side because we are concerned about the actions of" the protesters, said Nititorn Lamleu, leader of a pro-royalist group.