A report released by the group Licadho said the voters were "intentionally manipulated" into casting the fraudulent ballots, The Phnom Penh Post reported Monday.
In one alleged incident, Licadho said professors at a Phnom Penh university bussed hundreds of their students to a district some miles outside of the city.
Other violations occurred, the group alleged, when more than 100 workers at a rock quarry in central Cambodia were taken to a newly created polling station near the national capital to vote.
The owner of the quarry is allegedly a member of the ruling party.
More than 30 percent of the names on the voting list at that polling station were duplicates, compared to 2 percent nationwide, Licadho said.
Naly Pilorge, director of the watchdog group, said Licadho had expected "significant" voting issues "but the indications of vote rigging we saw went beyond that."
Tep Nytha, the National Election Committee's secretary-general, said the NEC had received the report, but "will not investigate" its allegations.
Koul Panha, head of the election watchdog group Comfrel, said Licadho's findings were "very consistent" with those of other groups, but added further investigation was needed "to know the scale of the problem."
Cambodia has more than 19,000 polling stations, Licadho said.
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