Two opposition activists were arrested this morning in Tbong Khmum province after they rode motorbikes with Vietnamese licence plates to a polling station.

Chan Moul Commune Police Chief Khuong Chhordarith confirmed Muon Tabeam, 30, and Roeun Rach, 25, were arrested at 8am.

Minister of Information Khieu Kanharith, in a post on his Facebook page, said the pair were planning a "dirty trick" by staging a photograph, with the alleged involvement of another activist, aiming to suggest foreigners had cast ballots in the election.

Chhordarith said authorities made a report and confiscated their motorbikes after he learned a picture had been taken, though he had seen no evidence of video or photographs.

"They are Khmer, they are not Vietnamese," he said.

"We arrested them after we learned that they were taking the picture and accusing that Vietnamese people had come to vote."

He said police were looking for the alleged photographer and were unaware if any image had been posted to social media.

He said the motorbikes belonged to them, and admitted that near the border around 40 percent of motorbikes bear Vietnamese plates.

Memot district is about 10 kilometres from the Vietnamese border.

The arrest was confirmed by Vainh Theal, Cambodia National Rescue Party (CNRP) first candidate in Chan Moul commune, Memot district, where the incident occurred.

Theal stressed neither man had taken a photo, and claimed both had merely borrowed motorbikes with Vietnamese plates in order to vote.

"He borrowed someone else's bike ... after he voted, his motorbike was confiscated," he said.

"The two have not taken any picture."

The opposition Cambodia National Rescue Party has long campaigned on deep anti-Vietnamese sentiment and has accused the ruling party of being Vietnamese puppets. During the 2013 national elections, riots broke out in Phnom Penh over suspicions that Vietnamese had voted illegally.

The current government, including Prime Minister Hun Sen, has had deep ties to Vietnam ever since its army, along with Cambodian defectors, overthrew the Khmer Rouge in 1979, installing a Vietnam-backed regime in its place.

Additional reporting by Erin Handley