The special cabinet meeting was attended by Deputy Prime Minister Pracha Promnok, Deputy Prime Minister Pongthep Thepkanchana, Interior Minister Charupong Ruangsuwan, Tourism and Sports Minister Somsak Purisrisak, PM's Office Minister Varathep Rattanakorn, Deputy Defence Minister Yutthasak Sasiprapa, police chief Adul Saengsingkaew, Lt Gen Paradorn, defence permanent secretary Nipat Thonglek and PM's deputy secretary-general Thawat Boonfueng.
Lt Gen Paradorn said that the anti-government protest by the People's Democratic Force to Overthrow Thaksinism (Pefot) justified the use of the security law.
Chaiyant Chaiyaporn, a political scientist at Chulalongkorn University, said the ISA will lose its sanctity and no longer hold a special status after being invoked against only a few hundred protesters.
"The government is over-reacting by imposing the ISA. Why not impose other ordinary laws. When special laws are invoked too often, they are no longer special," Mr Chaiyant said, adding that such a small number of protesters did not warrant the imposition of the ISA and such a huge police presence.
Lt Gen Paradorn said the invocation of the ISA was based on the recommendations of the Royal Thai Police and security agencies. National police chief Adul Saengsingkaew has been appointed as chief of the peacekeeping command.
Lt Gen Paradorn said security agencies have assessed the protest situation and believe it could become protracted and lead to a public disturbance.
He said security agencies also believed 56 civic groups planning a gathering at Rangsit University on Sunday are linked to Pefot. This could be a reason for Pefot trying to extend its rally, he said.
On Monday, Pefot members moved from Lumpini Park, where a small number of protesters had been encamped for about a month, to Government House. The government responded by instructing Metropolitan Police to close off the area.
The protesters rallied for the third day on Phitsanulok road on Wednesday. The road has been closed to traffic between the Panichayakarn and Suan Misakawan intersections. Sixteen companies of police have been deployed in the area.
The NSC chief said it was feared the protest might be protracted and that an unnamed "third party" could interfere. It had been reported that Pefot was asking 56 other anti-government groups to join the protest, he said.
Pol Gen Pracha said Prime Minister Yingluck Shinawatra, who is in Brunei attending the Asean Summit, had been bried on the situation.The prime minister replied that every step taken must be lawful.
It was reported that Pol Gen Pracha had signed an order approving the invocation of the ISA.
A meeting of the Internal Security Operations Command (Isoc) was subsequently called, chaired by Pol Gen Pracha, to approve an operational plan under the ISA for immediate implementation.
National police chief Pol Gen Adul Saengsingkaew has been appointed director of the peace-keeping command for the implementation of the ISA.
Chinese Premier Li Keqiang will make an official visit to Thailand from Friday to Sunday, Oct 11-13. He is scheduled to call on Ms Yingluck at Government House on Friday at 5pm.