Mr Suthep, the People's Democratic Reform Committee (PDRC) secretary-general, accused the government of Yingluck Shinawatra of trying to confuse the public about wanting to reform the country following a general election that has been called for Feb 2.
"At first, I was told that the government will hold talks on national reform and it wants the PDRC to join," he said in his Friday night address at the group's rally site at Democracy Monument in Bangkok.
"But I received telephone calls from respected senior figures telling me not to go because attending this meeting would mean that we affirm the government's legitimacy.
"In brief, if there is a general election before national reform, we won't negotiate because it would be a waste of time," said the former Democrat Party power broker.
Some other PDRC figures had indicated earlier that they would attend the government-sponsored event on Sunday. The PDRC is also taking part in a seminar on reform at Thammasat University on Saturday.
Mr Suthep, meanwhile, has informed Supreme Commander Tanasak Patimapragorn of the PDRC's intention to meet him at 3pm on Saturday at armed forces headquarters to explain what "the masses" want and to open up a two-way communication channel.
Once Gen Tanasak understands what the people want, the PDRC will not force him to take sides, said Mr Suthep.
However, he said, he was informed that the national police chief (Pol Gen Adul Saengsingkaew) would not join Saturday's meeting and that Pol Gen Adul planned to arrest him.
"I will travel with my PDRC friends without the masses (to the armed forces headquarters). We are fighters for the people so we have to be valiant and dignified. My brothers and sisters can wait for me here tomorrow (at the rally site)," Mr Suthep said.
The red-shirt United Front for Democracy against Dictatorship (UDD) has declined the military’s invitation to listen to the PDRC proposals.
UDD chairwoman Tida Tawornseth said her group would not send any representatives to take part in the meeting on Saturday.
Lt Gen Harit Pumhiran, head of the military's Directorate of Joint Civil Affairs, said military leaders would not ask any questions nor would they give any opinions during the forum.
He reiterated that representatives from state agencies, academics, foreign and Thai news media, and various other stakeholders were also invited to attend the meeting.
They include those from the Office of the Auditor General of Thailand, National Anti-Corruption Commission, Office of the National Human Rights Commission, Political Development Council, the Office of the Civil Service Commission, Lawyers Council of Thailand, and the Election Commission.