The Pheu Thai Party is expected to form a coalition with the Move Forward Party (MFP) as unofficial election results showed they were neck and neck in a tight race.
The Election Commission (EC) began counting votes after the general election concluded at 5pm on Sunday.
As of 10.30pm, two parties from the previous opposition bloc -- Pheu Thai Party and the Move Forward Party -- were taking it in turns in leading both the constituency and party-list races while counting votes was still being conducted.
The votes were still being tallied, with unofficial results expected to be available around 11pm, according to the EC.
Srettha Thavisin, one of Pheu Thai's three prime ministerial candidates, said that unofficial results showed Pheu Thai was still in the lead, not the MFP.
"I am still confident Pheu Thai will win the election,'' Mr Srettha said, adding that he and Pita Limcharoenrat, the MFP leader, had not discussed the formation of a government over the phone yet.
"We will discuss the matter with our allies which share democratic ideologies first,'' Mr Srettha said.
Asked whether he was concerned about allegations of poll fraud, Mr Srettha said he was, and the party had sent officials to observe counts at polling stations.
Asked about the unofficial results that showed MFP had taken the lead in all 33 constituencies in Bangkok, Mr Srettha admitted he was disappointed and a bit surprised. Asked if he was upset that Pheu Thai may not win the election by a landslide as it had hoped, he said he was not.
Prasert Chantararuangthong, Pheu Thai party secretary-general, said the party will form a coalition government with parties from the previous opposition bloc.
He said while the counting was still ongoing, Pheu Thai would allow the party that won the most seats to form a government first.
Earlier, Mr Pita on Sunday confirmed that it could form a coalition government with the Pheu Thai Party and other parties from the previous opposition bloc. He confirmed he would not do a deal with the military-linked parties.
Speaking after learning the result of exit polls, Mr Pita said MFP and Pheu Thai could have enough House seats to form a government. "This is a positive sign. It's time for Thailand to change," he said.
Asked about coalition talks with Pheu Thai, Mr Pita said he expected the talks to start at 10-11pm on Sunday night. "A collaboration of the (former) opposition parties will deal with the challenges that the country faces. We will change Thailand together," the MFP leader said.
Mr Pita, 42, said the formation of the next government would be subject to a memorandum of understanding and there must be conditions.
He ruled out the possibility of forming the next government with the United Thai Nation Party of Prime Minister Prayut Chan-o-cha and the Palang Pracharath Party of Deputy Prime Minister Prawit Wongsuwon.
Mr Pita did not rule out possible coalition participation by the Bhumjaithai Party of Deputy Prime Minister and Public Health Minister Anutin Charnvirakul.
Meanwhile, Deputy Prime Minister Wissanu Krea-ngam said that the incoming government is likely to face protests and resistance from its opponents.
He said while the economy will start to rebound and people's livelihoods will improve after the poll, protests and unrest are likely to resurface as society remains split along political and ideological grounds.
This will obstruct any effort to steer the country forward, the deputy prime minister said, adding that whichever party that will lead the government coalition must be prepared to deal with such resistance. "Risk factors remain [after the poll]," he said.
"Don't forget about the Yingluck Shinawatra government. It won an overwhelming number of House seats, but it later faced resistance and demonstrations," Mr Wissanu said. Asked whether the country will undergo big changes after the poll, Mr Wissanu said change always occurs after an election.
"Even if the same government returns to power after the poll, it must still change its policies, change its team, change the way it works," Mr Wissanu said. "Whoever forms the new government is unlikely to revert to the 'old ways'. They must learn from past mistakes and keep up with the changing circumstances," Mr Wissanu said.
Navy commander Adm Choengchai Chomchoengpaet said he hoped the post-election processes will go smoothly without any unrest. "Everyone wants to see democracy work and have a new government to lead the country to prosperity," he said.
National police chief Pol Gen Damrongsak Kittiprapas on Sunday said more than 150,000 police were deployed to maintain order at polling stations nationwide.
