The government is trying to control the proposed constitution drafting assembly (CDA) and create a charter that will weaken people's rights, the Democrat Party says.
Independent organisations and the courts could also end up with a reduced role and corrupt politicians could evade legal punishment, Democrat Party MP Nipit Intarasombat said.
Charter drafters are being elected in each province. Most drafters would be on the same side as the government, because the results of the election would reflect the government's support, he said.
Apart from the 77 drafters _ one per province _ to be elected at provincial level, the government will appoint 22 experts to the assembly and they will also come under government control, he said.
"As the CDA will be influenced by the government, three concerns arise: that they will attempt to control and weaken independent organisations, to control and weaken the Constitution Court, the Court of Justice and the Administrative Court, and to arrange an amnesty for those prosecuted by the ASC," Mr Nipit said.
He was referring to the now-dissolved Asset Scrutiny Committee (ASC) established after the 2006 coup that toppled the Thaksin Shinawatra government.
The ASC investigated allegation of corruption against the former prime minister.
A House committee has been scrutinising a proposed amendment to Section 291 of the constitution to set up the CDA.
Mr Nipit is in the minority group of the committee. The panel will propose the bill to parliament on Wednesday and Thursday for its second reading.
Appointed senator Somchai Sawaengkan said the House committee scrutinising the bill seemed to ignore criticisms of the proposals.
As many as 172 House members expressed reservations about the bill but the committee spent only one and a half days listening to them.
"The government may have already decided who will be the 22 appointed drafters," he said.
"The election of the 77 other drafters is based on the principle of excluding candidates who are defeated.
"By this means, the minority will not be represented in the CDA and the majority can then design the constitution."
Opponents deprived of their right to a say might have to take their reservations to the streets, he warned.
Jirayu Huangsap, deputy spokesman of the ruling Pheu Thai Party, said his party would meet today to discuss its stance on the bill's second reading. He expects the third reading to take place on April 24 or 25.