Leaders of the protests seeking to oust Thai Prime Minister Thaksin Shinawatra Monday have rejected an offer of a national unity government, saying the embattled premier is "insane".

Thai protesters demand the resignation of Prime Minister Thaksin Shinawatra at Thai Rak Thai headquarters in Bangkok. The leaders of the protests have rejected an offer of a national unity government, saying the embattled premier is "insane". [AFP]

Thaksin made the offer late on Sunday after weeks of opposition protests to demand he step down, saying he would invite his opponents to join a new government if he wins elections next weekend.

But Sondhi Limthongkul, a media tycoon and former ally of the premier who has turned into one of his harshest critics, said Thaksin only made the offer because he had no other option.

"He made the proposal because the protest gave Thaksin no choice," he said.

"Thaksin is insane. He proposed a government of national unity and invited us to join after he labelled everyone against him as the bad guys," Sondhi Limthongkul said on Thai television.

Another protest leader, Somskiat Pongpaiboon, also rejected the offer, saying: "We don't want to breed with Thaksin's Thai Rak Thai government."

Thaksin called the April 2 snap election three years early, in hopes of ending weeks of street protests demanding his resignation over allegations of corruption and abuse of power.

But the leading opposition have boycotted the polls, casting doubt on whether the election will meet legal requirements to form a new parliament.

The three parties boycotting the polls were meeting Monday to consider the proposal.

The offer came after opposition protesters staged their biggest rally yet, gathering some 100,000 people late Saturday to demand that Thaksin resign.