Media reports said preliminary results were expected to be announced later in the evening. The National Election Committee (NEC) will start announcing preliminary results at around 6:00 pm local time. Official results are expected in mid-August.
NEC said in a statement that the voter turnout was 82.71 percent, Presstv Reported.
Sik Bun Hok, the NEC chairman, praised in a televised news conference the high voter turnout, saying, "This is the success of the election."
"This time is higher. Cambodia should be proud," the chairman added.
Sik said the high voter turnout put to rest doubts about the election's legitimacy.
"This answers the international community's question about whether Cambodia loves democracy," the official said, adding that all polling stations in the country "operated without any obstruction."
Eight million and three hundred thousand people were registered to vote on Sunday.
Yoeung Sotheara, who previously worked as a poll monitor in Cambodia, said Sunday's result was best compared with the result of the nationwide local government elections in 2017, where the turnout was 90 percent.
In the previous general elections in 2013 the turnout was 69.61 percent.
Electoral reforms in 2016 made it easier for people to register where they lived and cleaned out dead people and "ghost" names from the voter list, he said.
Phil Robertson, deputy director of the Asia Division of Human Rights Watch, said many voters grudgingly went to the polls out of fear. Authorities have warned that anyone boycotting the vote will be seen as a "traitor."
Hun Sen, a former Khmer Rouge commander, has ruled Cambodia for more than 30 years and is the world's longest serving prime minister.
The 2018 election is the country's sixth since 1993, when the nation emerged from decades of war.
The elections were marred by voter intimidation and absence of any significant challenger to Hun Sen. Nineteen political parties ran against Hun Sen's ruling party, but none are strongly critical of the government.
Cambodia ruling party claims 'huge victory'
A few hours after vote counting started, Cambodia's ruling party claimed it had won a landslide victory in the election.
Cambodian People's Party (CPP) spokesman, Sok Eysan, told AFP late Sunday they expected to win "over 100 seats" in the 125-seat parliament, citing preliminary figures from the National Election Committee.
"The CPP will get more than 80 percent of the popular vote," he said, adding, "This is a huge victory for us."