In an exclusive interview with the Bangkok Post, Mr Suthep reiterated the party's position on the Worachai Hema amnesty bill and slammed Prime Minister Yingluck Shinawatra's proposed political reform assembly.
He said the government was merely attempting to serve deposed prime minister Thaksin Shinawatra's agenda.
"I can say for myself that I won't take part in this charade," Mr Suthep said.
"I don't believe or have any faith in it.
"I believe [Ms Yingluck] isn't sincere. The people in the government aren't sincere about implementing reforms. They are playing us, but we won't be played," he said.
The government says it intends to invite the Democrat Party today to join the reform assembly.
Mr Suthep, however, said if the party took part in the assembly, it would hand the government the legitimacy it was seeking.
The Democrats would not be left at a disadvantage in boycotting the political reform initiative, he insisted. The government would see for itself how many people oppose its amnesty plan, he said.
Asked about People's Alliance for Democracy leader Sondhi Limthongkul's call for the party's MPs to resign and join the fight against the bill, Mr Suthep said the party has obligations in parliament.
Several groups oppose the Thaksin Shinawatra regime. Each has chosen its own approach to suit its obligations and limitations, he said.
Mr Suthep said the Democrats will not intervene.
"It is important that we keep fighting [the bill] in parliament. If we resign, it is over," he said.
"They can push the bill in three straight readings. Many issues need to be scrutinised _ the government's two-trillion-baht infrastructure scheme, and the charter amendments, for example.
"How we will campaign [against the bill] will be discussed without hurrying. We have not reached a make or break point yet," he said.
Mr Suthep said the government is distorting facts by accusing the opposition of plotting to overthrow it.
If the Democrat Party was chasing power, it would have accepted the government's invitation to join the coalition, he said.
The amnesty bill, if passed, will overrule judicial power, he said.
The party will seek to topple the Pheu Thai-led government only when it is clear the government has destroyed the rule of law.
Critics have said the party's Aug 7 march to parliament appeared "half-hearted" as the Democrats dissolved the rally after police refused to allow the protesters access to restricted areas.
Mr Suthep defended the move and said the party did not intend to instigate public unrest.
They were not interested in torching property or taking the city hostage.
Mr Suthep said the party has agreed to wait until the bill passes all three readings before "blowing a whistle" to start a mass rally.
Some people are not aware of the government's hidden agenda while others believe the party still has plenty of time to oppose the measure.
The party will also lobby the Senate to block the bill and if that fails, it could turn to the Constitution Court.
"At the same time the Democrats' 'Stage of Truth' rally will be instrumental in the campaign to raise public awareness," he said.
"By the time the bill reaches the Senate, the public will be ready," he said.
He said the party might resort to multiple rallies, work stoppages, civil disobedience, boycotts of products associated with government figures, or a march to Government House.
"The Aug 7 march is proof that a peaceful demonstration can happen within the law and next time it might draw tens of thousands of people," he said.
Asked if a recent court ruling on the six deaths at Pathum Wanaram temple would affect the party's campaign against the amnesty bill, he said the court's ruling has established only that the six civilians were killed by security forces.
If the Department of Special Investigation exploited the ruling to charge him and Democrat leader Abhisit Vejjajiva with causing death and injury on the basis that the former Democrat-led government oversaw the military operations, he and the party leader would fight in court.
He denied the government any possibility of reaching a forced compromise.
"We insist that everyone should answer to the law; we and the protest leaders," he said. "If the government is serious about reconciliation, it must respect the law and let the justice process take its course.
"They can't use bullying tactics to force us to negotiate. We can't cooperate," he said.