Deputy Prime Minister Prawit Wongsuwan said on Monday he was not concerned about a student-led demonstration on Wednesday as protest leaders sought to escalate their push to demand a new constitution and oust Prime Minister Prayut Chan-o-cha.
During three months of protests, anti-government activists have also broken a taboo by calling for reforms of the powerful monarchy of His Majesty the King, who is "enthroned in a position of revered worship" according to the constitution.
Protesters, who drew tens of thousands of people to a demonstration last month, said they planned to gather on Wednesday at Bangkok's Democracy Monument before moving to Government House and would camp there overnight.
Gen Prawit told reporters that he did not expect huge turnout.
"We're prepared and not worried," he said. "I think we can handle it."
The protest leaders, organising under the new banner of the People's Movement, said their focus would be a call for constitutional changes before a parliament sitting on Nov 1.
"We also want to oust Prayut," said Panusaya Sithijirawattanakul, one of the leaders, adding that she expected even more people than at last month's protest in Bangkok.
Protesters say the constitution was engineered to ensure that Gen Prayut, who first seized power in a 2014 coup, continued in office after an election last year. He says the election was fair.
Some protesters also want a reduction in the King's powers to reflect Thailand's status as a constitutional monarchy.
Raising the prospect of an encounter between His Majesty and the protesters, his motorcade is due to pass Democracy Monument on Wednesday as he presides over a ceremony at a royal temple during a rare visit to Thailand.
Police said they would urge protesters to choose another location or at least clear the way for the motorcade.
Arnon Nampa, another of the protest leaders, said last week that demonstrators would not obstruct the motorcade but would show a three-finger salute - a symbol of resistance - if it passed by.
According National Security Council secretary-general Natthapon Nakpanich, some politicians are surreptitiously supporting the planned anti-government demonstration on Wednesday by helping protesters travel from 40-50 provinces to Bangkok, but there will probaby still be fewer than 10,000 people at the rally.
Gen Natthapon was responding on Monday to questions about government preparations for the demonstration.
He said Prime Minister Prayut Chan-o-cha had instructed the police force to monitor the situation and deal with the demonstrators.
Gen Natthapon quoted Gen Prayut as saying he did not want Thailand's success in dealing with Covid-19 to be imperiled by unrest.
It was not known whether the protesters would move to Government House or not, but authorities would do their best to keep the situation under control, he said.
Based on latest intelligence reports, it was estimated that fewer than 10,000 people would attend the rally.
"But we will not take this matter lightly, or underestimate them," he added.
Gen Natthapon said there would be some protesters from 40-50 provinces, but not in large numbers. Their travel to Bangkok would be arranged by the same groups of politicians as before, he said, and declined to be more specific.
So far, there had been no reports of involvement by third parties, he said, "but we will not be careless".
Asked about a report that the demonstrators planned to give a three-finger salute on seeing a royal motorcade passing by, Gen Natthapon said the Royal Thai Police Office would deal with this matter.
Organisers have called the demonstration to mark the Oct 14, 1973 popular uprising that resulted in the end of the military dictatorship of Field Marshal Thanom Kittikachorn.
