The former Khmer Rouge fighter turned prime minister appeared so confident of victory that he did not even bother to personally campaign.
The opposition decried what it described as the kingdom's worst ever vote irregularities.
"The situation is more serious than at any previous election," Cambodian National Rescue Party spokesman Yim Sovann told AFP.
He said many names had disappeared from the electoral roll while thousands of people could not vote as someone else had already used their ballot.
Hundreds of people protested at one polling station in eastern Prey Veng province against alleged irregularities, a witness told AFP by telephone.
But the National Election Committee denied the accusations.
"There is no problem of names missing from the lists," NEC secretary general Tep Nytha told AFP.
Even before polls opened, the opposition had warned that 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.
Rainsy toured polling stations in the capital Phnom Penh on Sunday to "collect more evidence" of vote irregularities.
He said that if indications pointed to a "plot to rig the election" then "definitely we will protest".
The CNRP said earlier that it had uncovered irregularities such as tens of thousands of duplicated voter names that would allow some people to cast ballots twice.
Local poll monitor the Committee for Free and Fair Elections in Cambodia alleged that up to 1.25 million people who are eligible to cast ballots are not on the voter lists.
Early results were expected later today.
You’ve reached your limit of {{free_limit}} free articles this month.
Subscribe now for unlimited access.
Already subscribed? Log in
Subscribe to read the full story →
Smart Quarterly
₹900
3 Months
₹300/Month
Smart Essential
₹2,700
1 Year
₹225/Month
Super Saver
₹3,900
2 Years
₹162/Month
Renews automatically, cancel anytime
Here’s what’s included in our digital subscription plans
Exclusive premium stories online
Over 30 premium stories daily, handpicked by our editors


Complimentary Access to The New York Times
News, Games, Cooking, Audio, Wirecutter & The Athletic
Business Standard Epaper
Digital replica of our daily newspaper — with options to read, save, and share


Curated Newsletters
Insights on markets, finance, politics, tech, and more delivered to your inbox
Market Analysis & Investment Insights
In-depth market analysis & insights with access to The Smart Investor


Archives
Repository of articles and publications dating back to 1997
Ad-free Reading
Uninterrupted reading experience with no advertisements


Seamless Access Across All Devices
Access Business Standard across devices — mobile, tablet, or PC, via web or app
