(Reuters) - Thailand's opposition Democrat Party said it will challenge a disputed weekend ballot in court on Tuesday, while the Election Commission probed possible campaigning irregularities in a drawn-out political conflict that showed no sign of ending.

In a further blow for caretaker Prime Minister Yingluck Shinawatra, China pulled out of a deal to buy 1.2 million tonnes of Thai rice amid a corruption probe, the commerce minister said, adding to the financing problems of a subsidy scheme that had helped win her huge rural support.

The Democrats, who boycotted the election, will file two complaints with the Constitutional Court, spokesman Chavanond Intarakomalyasut said.

"The first regards the election directly. We will argue that the election violated the constitution, in particular article 68 which prohibits people from undermining the constitutional monarchy and trying to grab power through unconstitutional means," he said.

"In a separate petition, we will file for the dissolution of (Yingluck's) Puea Thai Party for announcing the state of emergency which meant the election could not be held under normal circumstances."

Anti-government protesters have been on the streets since November, saying Yingluck must resign and make way for an appointed "people's council" to overhaul a political system they say has been exploited by her billionaire brother, former premier Thaksin Shinawatra.

The protesters, who accuse Yingluck of being Thaksin's puppet, draw their support from Bangkok's middle-class and elite, as well as the south. The Shinawatra power base is among the mainly rural poor in the populous north and northeast.

Yingluck imposed a state of emergency last month to try to control the protests, allowing security agencies to impose curfews, declare areas off-limits and detain suspects without charge. It appears such measures have not been enforced.

Good money after bad

Sunday's election was generally peaceful, with no repeat of the chaos seen the previous day when supporters and opponents of Yingluck clashed in north Bangkok. Whatever the result, it is unlikely to change the dysfunctional status quo after eight years of polarization and turmoil.

The Election Commission said it was looking into complaints regarding alleged abuse of authority by the government during Sunday's vote.

It is due to meet on Wednesday to discuss problems surrounding the election, including the failure to register candidates in 28 electoral districts after protesters blockaded candidate registration centers in December.

Thailand's anti-graft agency has accelerated an investigation into Yingluck's role as head of a rice price-support scheme that has cost taxpayers billions of dollars, leading to China cancelling the deal.

"China lacks confidence to do business with us after the National Anti-Corruption Commission started investigations into the transparency of rice deals between Thailand and China," Commerce Minister Niwatthamrong Bunsongphaisan told reporters.

He said the government would open a tender to sell 400,000 tonnes of rice from state stocks next week. It is desperate to get funds for the scheme because some farmers who have sold grain to the state have been waiting months for their money.

About 500 employees of state-owned Krung Thai Bank Pcl protested outside the bank's headquarters on Tuesday, urging it not to support the rice scheme with loans, saying it threw good money after bad.

The bank said it wouldn't proceed.

Midnight at the oasis

The anti-government protesters closed two camps on Monday and marched to a third, the green central oasis of Lumpini Park. In one bit of good news for Yingluck, their numbers appeared to be significantly lower on Tuesday. Small groups milled around the grass after spending the night in tents.

The demonstrators blocked balloting in a fifth of constituencies on Sunday. The election is almost certain to return Yingluck to power and, with voting passing off peacefully across the north and northeast, Yingluck's supporters will no doubt claim a legitimate mandate.

But there was no indication of when re-runs of disrupted ballots would be held or when the commission would be able to announce a result.

The protesters say former telecoms tycoon Thaksin has subverted a fragile democracy with populist politics such as the rice subsidies, cheap loans and healthcare for the poor that have won his parties every election since 2001.

Thaksin's critics also accuse him of disrespecting Thailand's revered monarchy, which he denies.

Thaksin has lived abroad since 2008 to avoid a jail term for a graft conviction he says was politically motivated.

His supporters accuse the military and the establishment, including the judiciary, of colluding over the years to oust his governments.

The military, which has staged numerous coups since Thailand became a constitutional monarchy in 1932, overthrew Thaksin in 2006, but this time has stayed in the wings.