PHNOM PENH, Cambodia — Cambodia’s opposition party claimed an upset victory Sunday in an election the longstanding incumbent had been expected to easily win, but called on supporters to wait for the official results.

The claim, made in an emailed statement by the Cambodia National Rescue Party, was impossible to immediately verify, as no results had yet been released by the authorities.

At a news conference after the statement was released, party leader Sam Rainsy was less clear about whether the claim was that his party had actually won a majority at the polls, stressing that it was a victory for the country.

There was no immediate reaction from the ruling Cambodian People’s Party of Prime Minister Hun Sen, who has ruled Cambodia for nearly three decades.

An opposition victory would be an astonishing surprise. On Saturday, Rainsy had virtually acknowledged that his party would lose the election, saying the polls were unfair but that his battle for democracy would go on.

Rainy has a reputation for provocative statements that have sometimes landed him in trouble. His claim, coming before the release of official results or any statements from the ruling party, could be seen as a pre-emptive move against Hun Sen’s side stealing the election.

It also sets up the possibility of serious post-election confrontations. Hun Sen, in statements suggesting there would be chaos if he lost, had implied he would not accept an opposition victory.

Im Suosdey, president of the government-appointed National Election Committee, said the first official results would be released at 8:30 p.m. GMT.

“Today is a historical day when the CNRP won the parliamentary elections for the fifth mandate period in 2013,” said the party’s statement, emailed about 3½ hours after the polls closed at 3 p.m. “CNRP wishes to thank all compatriots heartily that voted for the CNRP so that the party won these elections.”

“CNRP wishes to appeal to all compatriots to maintain calm and order to wait for the official election results that will emerge soon and appeal to all compatriots to avoid to use violence or to create any trouble that could cause social unrest,” the statement said.

Hun Sen, who has been in power for 28 years, cast his ballot just after the polls opened near his home in Takhmau, just south of the capital, Phnom Penh.

Rainsy began his day with a visit to a polling station near his party’s office in Phnom Penh. Hundreds of voters, particularly younger ones, greeted him enthusiastically, with some of the bolder ones hugging him.

Counting of the paper ballots at polling stations began shortly after the polls closed, with representatives of political parties witnessing the process.

The general election was Cambodia’s fifth since 1993, when the United Nations helped stage the country’s first free polls since the 1975-79 genocidal rule of the Khmer Rouge and a subsequent period of civil war and one-party rule.

There were many reports of voting irregularities on social media from local journalists and election watchdog groups, but an overall picture was unlikely to emerge until Monday at the earliest.

Passions boiled over at one polling station in Phnom Penh’s Stung Meanchey district, where Associated Press reporters saw two official vehicles that had been destroyed by rioters. Military police, who in Cambodia are used in a civilian capacity, were deployed to restore order. The unrest was originally thought to be related to people not being allowed to vote, but other accounts said a brawl broke out when a crowd confronted a man they believed to be ethnic Vietnamese trying to vote without the right to do so.

Rainsy said his party would wait before deciding what to do about the alleged irregularities, but added that if it was clear the voters’ will was being denied, “definitely, there will be protests.”

The streets around Hun Sen’s residence in Phnom Penh were closed Sunday, with police patrolling in an apparent effort to ward off protesters.

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