Security agencies held talks with Prime Minister Yingluck Shinawatra and cabinet ministers at Government House on the eve of the demonstration to evaluate the situation.
NSC secretary-general Paradorn Pattanatabut said after the meeting that protesters who are mainly from Bangkok are being organised by "a political party pulling their strings". He was referring to the opposition Democrat Party, which publicly announced the Samsen demonstration near its head office will start at 6pm Thursday.
Lt Gen Paradorn refused to put an estimate on the size of the turnout expected for the protest, but said security authorities remain confident in their ability to control the rally. The protesters were not expected to move toward parliament at this early stage.
The rally is being held to coincide with the second reading of the revised amnesty bill in the House of Representatives.
The House committee vetting the bill altered the legislation from its original version, so it now provdes a blanket amnesty to all those involved in political unrest from the 2006 coup until Aug 8, 2013, including convicted former prime minister Thaksin Shinawatra. The old version of the bill only proposed amnesty to protesters and members of the general public, but excluded protest leaders, politicians and officers responsible for ordering the crackdown on the anti-government street demonstrations.
The NSC chief urged the public not to join the protest, saying the controversial bill is still up for debate through the parliamentary process that will see the legislation further scrutinised in the House and the Senate.
"The administration needs to explain the steps of the parliamentary process to the public,'' the NSC chief said, adding that the bill could still be referred to the Constitution Court.
The security meeting ruled out the expansion of the Internal Security Act (ISA) to include Phaya Thai, the area where the rally will be held. The ISA is currently enforced in three districts of the capital - Dusit, Phra Nakhon and Pomprap Sattruphai - in the immediate vicitinity of the parliament and Government House.
"No. There will be no (ISA) announcement in other areas,'' Deputy Prime Minister Pol Gen Pracha Promnok told reporters.
Suriyasai Katasila, coordinator of Green Politics, urged members from the provinces to start travelling to Bangkok to join demonstrations at Uruphong or Lumpini Park, where anti-amnesty rallies are already being held, because the authorities were likely to try and block them from making their way to the capital.
But the spotlight will be on Samsen.
Suthep Thaugsuban, a Democrat MP for Surat Thani, read out a party statement announcing the protest at a press conference in front of the statue of King Rama 7 outside parliament on Wednesday.
"The party wants to declare its intention to join people throughout the country to oppose the amnesty bill. Thai people nationwide are hereby invited to join the rally against the bill at Samsen railway station on Oct 31, starting at 6pm.
"The party, people and businesses around the country will continue to rally until the government agrees to drop the bill," Mr Suthep said.
Four Democrat Party executives and deputy leaders resigned their party posts on Wednesday so they could give their full attention to leading the protest. They are Issara Somchai, Korn Chatikavanij, Thaworn Senneam and Siriwan Prasjaksattru. All will continue their roles as MPs.
Mr Issara said the four had decided to resign because if they take part in the rally while holding executive party positions, their opponents might petition the Election Commission to dissolve the Democrats.
Democrat leader Abhisit Vejjajiva said he will be in parliament and decided not to join the rally in Samsen on Thursday.
Mr Suthep and Mr Abhisit must report to the Office of the Attorney-General (OAG) on Thursday to formally hear charges relating to the 2010 protests crackdown, after the OAG decided on Monday to indict the two. Mr Abhisit was prime minister and Mr Suthep his deputy at the time of the 2010 violence. Mr Suthep also chaired the Centre for the Resolution of the Emergency Situation in lieu of Mr Abhisit.
Meanwhile, the V For Thailand group issued a statement calling on its supporters to join the Samsen rally.
The statement said it was now time to rid society of bad politicians.
V For Thailand said medical and nursing students from various hospitals would gather in front of the Office of the Foundation for the Blind opposite Ramathibodi Hospital on Rama 6 road at 5pm on Thursday before marching to show moral support for anti-government protesters at Uruphong intersection.
Prime Minister Yingluck admitted on Tuesday she is concerned about the protest situation. She called on groups opposing the amnesty bill to stage peaceful and lawful rallies and avoid blocking roads.