Large crowds are expected at the Office of the Attorney-General (OAG) Wednesday following its move to summon suspects, including former protest leader Suthep Thaugsuban, over street demonstrations against the Yingluck government in 2013-14.
Security will be tightened with more than 50 people, including Mr Suthep, scheduled to hear indictment decisions.
Viroon Chantananan, of the OAG's Department of Special Litigation 4, earlier sent a letter to the Department of Special Investigation (DSI) asking for security to be beefed up at the OAG head office.
The charges against the suspects, which include insurrection, illegal assembly and obstructing elections, are in connection with the 2013-14 street demonstrations directed against the Yingluck Shinawatra administration that were led by the People's Democratic Reform Committee (PDRC). The protests were ended at gunpoint by the coup in May of 2014.
Prosecutors have also asked for two vans to transport indicted suspects to be arraigned later at the Criminal Court.
Other key suspects include several former Democrat MPs, activist monk Phra Buddha Isara, and academic Chirmsak Pinthong.
The prosecutors also met the DSI Tuesday, along with senior police officers from Metropolitan Police Division 2 and representatives from the Criminal Court to make preparations.
A source at the Criminal Court said preparations are in place with police and the military agreeing to send 30 officers each to help tighten security.
About 100 security officials are expected to be deployed at the court.
Bail applications will be considered by a committee and the criteria for consideration include the severity of the charges and the flight risk posed by each.
The suspects are required to report to prosecutors at 9.30am and anyone who is indicted will then be taken to the nearby Criminal Court.
According to a source, prosecutors have wrapped up the cases against the suspects and are ready to announce the decisions.
"If all the suspects show up, we'll know who will be indicted and on what charges and who won't," the source said.
"Those who are indicted will be taken to the court."
Asked about reports that nine of the suspects will be indicted, the source added the nine individuals have confirmed they will report to prosecutors. He did not divulge further details.
Mr Suthep and nine former PDRC protest leaders have reportedly asked their lawyers to make preparations to apply for bail if they are indicted.
Some academics may also seek a postponement claiming they are scheduled to attend other engagements.
Former PDRC spokesman Akanat Promphan said Tuesday he would report to prosecutors as scheduled.
Initially there were 58 suspects in the case but prosecutors decided to drop charges against one.
Of the remaining 57, four have already been indicted while seven others were categorised in a separate case because they faced less serious charges.