The run-up to Thailand's weekend vote was "heavily tilted" to benefit a party close to the ruling military junta, an Asian election monitor says.
The Bangkok-based Asian Network for Free Elections (ANFREL) criticised a messy ballot-counting process that created mistrust.
Two days after Sunday's vote, confusion remains as a pro-junta party and an opposition party both claimed victory.
The uncertainty diminished hopes that the first election since the 2014 military coup would end political turmoil in Southeast Asia's second-largest economy.
"The environment at large is heavily tilted to benefit the military junta," ANFREL official Amael Vier told a news briefing.
"A lot of people still express distrust towards the electoral process."
Thailand's Election Commission was not immediately available for comment. It has previously declined to comment on accusations of cheating.
With only partial results reported, the party backing junta leader Prayuth Chan-ocha has said it is gathering coalition partners to form a government.
But the main opposition Pheu Thai party, loyal to ousted former premier Thaksin Shinawatra, has alleged "irregularities" while also saying it is putting together a coalition government.
It could be days or even weeks before it is clear whether either has won enough seats to form a stable government.
Unofficial results for 350 directly elected "constituency seats" in the House of Representatives released on Monday by the Election Commission showed Pheu Thai leading with 137 seats, versus 97 seats for Prayuth's party.
But official results, including 150 more "party seats", would not be available until May 9, the election body said.
A fuller picture could emerge on Friday, when the election commission releases vote totals for each constituency, used to determine the allocation of party seats, in a complex formula.
The commission has blamed delays and irregularities in early partial results on "human error."
Since 2004, Thailand has been racked by street protests organised by both opponents and supporters of Thaksin, occasionally spilling into violence.
Parties linked to Thaksin have won every election since 2001, but the populist telecoms billionaire was thrown out by the army in 2006, and a government led by his sister was ousted in 2014.
Australian Associated Press