The ruling party of Cambodian Prime Minister Hun Sen claimed victory in Sunday's elections.
Although official results had yet to be announced, the prime minister's Cambodian People's Party (CPP) said it expected to win 68 out of 123 seats in the lower house.
"We can say we've won this election," CPP spokesman Khieu Kanharith told AFP.
The CPP had 90 seats in the previous parliament, so if confirmed the result would mark the loss of more than 20 seats.
If the figures are confirmed, they would represent a significant gain for the opposition and a setback for Hun Sen, who has served as prime minister since 1985.
The opposition decried what it described as the kingdom's worst ever poll irregularities, including missing voter names and thousands of people who turned up to find someone else had used their ballot.
"The situation is more serious than at any previous election," Cambodian National Rescue Party (CNRP) spokesman Yim Sovann told AFP.
The opposition caused brief confusion after claiming it had won the polls but it quickly retracted the statement. It had no immediate response to the CPP's later declaration of victory.
Protests broke out at one polling station in the capital Phnom Penh where a crowd destroyed two police cars, military police spokesman Kheng Tito said, after anger erupted over names missing from the voter list.
Rights groups also expressed concern that the ink used to mark voters could be easily washed off.
The National Election Committee denied all claims of irregularities.
Before polls opened, the opposition had said a Hun Sen win would be "worthless" without the participation of its leader Sam Rainsy.
The French-educated former banker returned to Cambodia on July 19 from self-imposed exile after receiving a surprise royal pardon for criminal convictions which he contends were politically motivated.
But he was barred from running as a candidate since the authorities said it was too late to add his name to the electoral register.
Rainsy said that if indications pointed to a "plot to rig the election" then protests would definitely happen.