Less than 10,000 people are expected at the pro-democracy demonstration in Bangkok on Wednesday, the secretary-general of the National Security Council, Nattapon Nakpanich, said on Monday.
Citing the latest intelligence reports, Gen Nattapon said Wednesday's rally will be attended by demonstrators from 40-50 provinces across the country.
He said that some politicians were involved in organising the rally, but didn't give any names.
So far, no information has been received about any attempts to stir up unrest at the protest, he said.
The Student Union of Thailand, meanwhile, urged students in all parts of the country to demonstrate their power by skipping classes on Wednesday and joining the gatherings on Ratchadamnoen Klang Avenue and outside Government House.
The union said the government's mismanagement of the country -- likening them to a dictatorial regime -- should prompt all students to come out and demand changes, including reform of the monarchy.
Thammasat University's Student Union on Monday called on the university to suspend classes from Wednesday until Friday to allow students to go to the demonstration.
Police said they will deal with the protesters if they camp out in front of Government House.
Authorities, however, won't allow protesters to cross Chamaimaruchet Bridge, which leads to the Government House's main entrance.
Another police commissioner said he was optimistic the protesters won't disrupt a royal motorcade which is scheduled to take place on the same day.
Police, he said, will do what it takes to prevent the rally from affecting the convoy, including negotiating with the protesters.
