"The EC legal team is preparing to sue Suthep if his comments turn out be malicious," EC secretary general Ekachai Warunprapha said.

Ekachai said EC chairman Vasana Puemlarp had not given a special instruction on the allegation, hinting that the case would be treated as a routine matter.

Ekachai's deputy, Pokkrong Sunthornsut, said Suthep should release his evidence rather than make verbal attacks against the EC.

Pokkrong said his office had cooperated fully with the Democrat Party to release membership records of the small parties and the favour should be returned in regard to the alleged involvement of EC officials in any attempt to rig party memberships.

Suthep, meanwhile, threatened to reveal how the ruling Thai Rak Thai Party contracted a small party to contest the April 2 election, which is considered a violation of campaign law punishable with party dissolution.

"Sue me, because I too want the truth to come out," he said, reacting to a threat of litigation from the ruling party to silence him.

Suthep said he would parade evidence of the electoral scandal at the Democrat rally, scheduled for Friday at Sanam Luang - if the ruling party failed to initiate court action against him.

He singled out three ruling party executives - Thamarak Isarangura, Pongsak Ruktapongpisal and Prommin Lertsuridej - for possible involvement in the deal.

The ruling party paid the Pattana Chart Thai Party and all of its candidates to contest the poll, he alleged.

Suthep said he would have to present his case at the Democrats' rally because he was not empowered to initiate legal action due to his party's boycott of the election.

Even though caretaker Prime Minister Thaksin Shinawatra denied his party's involvement, Suthep cautioned him to review the evidence before siding with his subordinates.

"Thaksin should brace himself for the facts, which might seriously affect his future and that of his party," he said.

Pattana Chart Thai leader Boonthaweesak Amornsin has denied the claims, but Suthep said the evidence would speak for itself.

"I have no intention of frame-up charges against Boonthaweesak. I want only to expose Thaksin's flawed leadership which tampers with the law to cling to power," he said.

He dismissed speculation that his party, the main opposition to Thaksin, was trying to frame the ruling party.

Prommin said he and the two other Thai Rak Thai executives, Thamarak and Pongsak, would take legal action against Suthep separately.

Thamarak said Suthep was trying to discredit the ruling party. He said he supervised his party's campaigning in the Northeast and had no reason to cheat.

In the last general election, the ruling party won 126 of the 136 House seats from the Northeast and hence had no reason to do a deal with minor parties, he said.