Ong-Ard Klampaiboon, spokesman for the Democrat Party, which is boycotting next weekend's polls because it says they cannot be neutral or fair, said on Monday: "I think this is Thaksin's new joke.
"His point is only to have the election and not to worry whether it is a dirty election or not."
Thaksin, who called the election three years early to foil a six-month street campaign to oust him, said late on Sunday that his opponents would have senior positions in a national unity government even if they took no part in the election.
There was no immediate reaction from the Matachon party, which is also staging a boycott of the election, but Chart Thai party officials rejected the olive branch from a prime minister they accuse of corruption and abuse of power.
Somsak Prisnanantakul, Chart Thai's deputy leader, told a radio station in Bangkok: "It's Thaksin's mirage. He's already rotten and he's trying to drag everyone in to be rotten like him."
Also on Monday, a small bomb containing TNT was found at the headquarters of the Democrat Party headquarters and police evacuated the building before defusing the bomb, a party spokesman said.