The opposition hopes the elections will be a springboard for more gains in a general election next year, when Hun Sen aims to extend his more than three decades in power.
Since the vote results were announced, pro-government groups have stepped up attacks on many of the newly elected officials, especially in social-media posts, in what the opposition sees as a deliberate smear campaign.
CNRP candidate Srun Chanty, 42, who won in Poipet town near the border with Thailand, said a picture of her was posted on Facebook accusing her of being Vietnamese.
Anti-Vietnamese sentiment is pervasive in Cambodia, even among progressive political groups, and political parties of all stripes often play the anti-Vietnam card against opponents to undermine their legitimacy.
"It won’t be easy working with this government, under this ruling party," Srun Chanty, adding that she worried about her security.
Other opposition members have been accused of being members of the Khmer Rouge, under whose 1975-1979 rule about two million Cambodians were killed or died of starvation.
Another newly elected opposition official was accused of being a "sword-wielding gangster" while the pro-government "Fresh News" web site accused others of running an illegal lottery racket.
Government spokesman and CPP member Phay Siphan denied that there was any government or ruling party backing for such intimidation.
"These attacks are not our official position," he said.
'GRUDGE'
An internal CPP memo seen by Reuters contained a suggestion that government infrastructure funding to an area that voted heavily for the opposition be cut, to show people they would suffer if they voted the wrong way.
Phay Siphan dismissed the leaked memo as "invalid".
"This is not the policy of the CPP or the government," he told Reuters.
Monovithya Kem, the CNRP's deputy public affairs director, denounced what she called "misinformation and discriminatory information" against the opposition, which she said she feared would only get worse.
"Obviously, it shows a grudge against our newly elected officials," she told Reuters.
"This smear campaign will only backfire, as all their smear campaigns have in the past. People chose to replace CPP with CNRP officials for many reasons."
Human rights groups fear a surge of political violence and other abuses in the run-up to the general election. Scores of opposition politicians and rights activists have been jailed in recent months.
The government denies accusations of rights abuses and Hun Sen has warned of chaos and even a revival of civil war if he were to lose the general election.
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