Human rights groups have condemned the vote as as a sham, while the United Nations has also questioned its validity.
The National Election Committee said voter turnout was 70.41 percent but this was challenged by witness reports in the country’s capital Phnon Penh.
One Dutch tourist, who wished to remain anonymous said he did not realise it was election day.
He said: ”Is it election day? I didn’t know. Seems very quiet, like a normal Sunday.”
Preliminary results are expected at 6pm local time (12pm BST).
But the official results will not be announced until mid-August.
Who will win Cambodia election?
Prime Minister Hun Sen, who has been in power since 1985, is believed to have already won the election in the absence of any real viable opposition.
Nineteen political parties ran against Hun Sen’s ruling Cambodian People’s Party (CPP) but none are critical of the government.
The former Khmer Rouge leader, 65, who became prime minister at the age of 32, has been described by his opponents as a dictator who stamps out dissent.
The main opposition, Cambodian National Rescue Party (CNRP), was dissolved last year and its leader, Kem Sokha, jailed on charges of treason.
CNRP narrowly lost Cambodia’s last general election in 2013, when a band of younger voters tried to bring about change against Hun Sen.
Voter turnout that year was 70 percent but CNRP lost the election and waster accused by CPP of attempting to overthrow Hun Sen’s rule.
Last year, 30 radio stations were closed down along with the daily English-language newspaper Cambodia Daily.
Today’s election is seen as a crushing blow to democracy by many.
Phil Robertson, deputy director of Human Rights Watch’s Asia Division, told Reuters in an e-mail many voters did go to polling stations out of fear.
One man, who asked not to be identified, said: “I did not want to vote but I did vote.”
On Friday, Cambodia blocked international news agencies from accessing independent media websites.
Cambodia’s National Election Committee website has been repeatedly down with an error message on display.