Prime Minister Yingluck Shinawatra called Sunday's snap election in a bid to defuse mass rallies that have dogged her government for three months, sparked deadly violence in Bangkok and thrust the kingdom into prolonged political turmoil.
The latest legal move by the opposition, which plans to file a legal challenge to the polls in the Constitutional Court on Tuesday, came as the US warned against any moves to stage a military coup.
"We certainly do not want to see a coup or violence," US State Department spokeswoman Jen Psaki said after opposition protesters prevented voting at thousands of polling stations on Sunday, prompting election authorities to withhold results until ballots are cast in all constituencies.
A lawyer for the opposition Democrat Party said their complaint to the Constitutional Court will pivot on a state of emergency, announced in the run-up to polls because of worries over political violence after a spate of gunfights and grenade attacks at protest sites.
The Democrats claim the decree also sought to muzzle critical media and promote pro-government news outlets, breaching the constitution.
The country's bitter polarisation hinges on the influence of Yingluck's older brother Thaksin, who lives in self-imposed exile yet draws adoration by the rural northern poor but is loathed by the Bangkok middle class and southerners.
The billionaire-turned-politician and his allies have won every election since emerging on Thailand's political stage more than a decade ago.
He was deposed in a military coup not long after his second electoral win in 2006 and two elected Thaksin-backed parties were subsequently dissolved by the courts.
Yingluck's government now faces a slew of legal challenges, including over alleged corruption linked to a controversial rice subsidy scheme and an effort to remodel the make-up of Thailand's senate to make it fully elected.
Observers say Thailand's powerful army is reluctant to step in this time, despite calls from anti-government protesters for it to resolve the crisis.
