Prime Minister Hun Sen’s personal website was hacked by members of the group Anonymous on Saturday, its homepage replaced with a video showing graphic images of dead bodies and acts of state-sponsored violence in Cambodia over the past 30 years.

In a Facebook post on Saturday afternoon, Mr. Hun Sen said the site was hacked at 4:12 a.m. in an effort “to destroy his reputation.” The website was restored later on Saturday and was functioning normally as of last night.

“This act was evidently and undeniably committed by opposition groups that always try to find wicked ways to attack the prime minister,” the prime minister wrote on his Facebook page. He did not name the opposition groups.

As part of the hack, the homepage of the website, which was launched in January and promotes Mr. Hun Sen’s official activities, was replaced with a doctored image of the prime minister wearing a large gold crown bearing the Facebook logo and accompanied by the words “King of Facebook”—an apparent reference to the premier’s ongoing online popularity battle with exiled opposition leader Sam Rainsy.

A YouTube video posted beneath the image, which was uploaded to the streaming site on Thursday, shows a different version of the hacked homepage featuring photographs of attacks and atrocities, some linked to Mr. Hun Sen during his three decades in power.

The photos include images of factional fighting in 1997, attacks on protesters following the disputed 2013 election, the deadly shooting of demonstrators in early 2014 and the beating of opposition lawmakers outside the National Assembly last year.

Others show violent land evictions, trucks packed with illegally logged wood, and the dead bodies of assassinated union leader Chea Vichea and slain environmental activist Chut Wutty.

A voiceover described the hack as the “first slap” in a campaign against Mr. Hun Sen, warning that two more would occur if he did not renounce the violence in which, the narrator claims, the prime minister is implicated.

While the Anonymous logo—a Guy Fawkes mask—appears twice in the video, nothing was posted to the Facebook or Twitter accounts of the hacking group’s Cambodian arm, which rose to local prominence when its members hacked more than 30 government and private-sector websites following the 2013 election.

Niklas Femerstrand, a Phnom Penh-based hacker and internet security researcher, said the group’s fluid membership would make it difficult to identify the individual behind Saturday’s hack, which was likely made easier because the software running the website—Joomla—was not updated following the discovery of a security flaw in the program in December.

“I very much doubt they used a sophisticated exploit or vulnerability,” he said. “The government is to blame for not updating their software.”

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Correction: A previous version of this article incorrectly stated that Niklas Femerstrand said the prime minister’s website was being run using software called Hoolahoop. The software is called Joomla.

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