Exit polls suggest that the party allied to Thailand's ousted PM Thaksin Shinawatra has taken a strong lead in general elections.
Samak Sundaravej's PPP is expected to win but not outright
But emerging partial results suggest the People Power Party (PPP) will fall short of an outright majority, and a coalition government is likely.
The pattern may change again, given that early results tend to come in from urban areas where the PPP is weaker.
It is the first election since the 2006 coup that overthrew Mr Thaksin.
Three-quarters of the ballots have been counted nationwide but although the exit polls point to a PPP win, they vary widely on the number of seats the party is expected to take.
Full unofficial results are expected to be announced at about midnight (1700 GMT).
Election monitors say that voting has mostly proceeded smoothly and been well-organised, despite complaints of vote-buying and other irregularities.
The BBC's Jonathan Head says a PPP win would be a setback to the military, which has tried to counter Mr Thaksin's allies.
Our correspondent says that if the PPP has not won an outright majority, there will be considerable pressure on middle-ranking parties to form a coalition with the PPP's main rival, the Democrats.
But he says this could lead to further instability as a multi-party government could well prove weak and short-lived.
Mr Thaksin himself has been in exile since the coup, and he and his Thai Rak Thai party were banned from politics by the military government.
But his allies have resurfaced under the PPP banner, and analysts say they have benefited from Mr Thaksin's populist appeal, especially in the countryside.
One voter, Roongchai, told the BBC that he liked Thaksin because he was "brave enough to change things".
"He might have made mistakes, but overall he brought positive change."
Return from exile?
Surapong Suebwonglee, secretary general of PPP, said the poll was "a victory for people and democracy".
"It shows that the coup one-and-a-half years ago has not benefited the country or anyone," he told the AFP news agency.
The PPP's right-wing leader Samak Sundaravej, 72, says he expects Mr Thaksin to return to Thailand from self-imposed exile in the UK if PPP wins an outright majority.
If he does return to the Thailand, Mr Thaksin will have to answer a number of corruption charges levelled against him in the courts.