HANOI (AFP) - Vietnam on Wednesday condemned what it called an attack by Cambodian activists on Vietnamese police and citizens living near the two countries' border, which left more than a dozen people injured.

Some 250 Cambodian activists, including a lawmaker with the opposition Cambodian National Rescue Party (CNRP), illegally entered Vietnamese territory in the southern province of Long An over the weekend, the Foreign Ministry in Hanoi said.

"Vietnamese security officers and local residents tried to reason with the activists and stop them, but they were attacked... seven Vietnamese were injured," it said in a statement released on Wednesday.

Cambodia's opposition party has long been critical of Phnom Penh's cosy ties with Hanoi and routinely accuses Vietnam of taking Cambodian territory along the 1,270km border, which is not fully demarcated.

Rhetorical and sometimes physical attacks by opposition lawmakers and activists on the Vietnamese border have become a common occurrence in recent years.

Vietnam "strongly condemned these violent acts" that could jeopardise the good relationship between the two countries, ministry spokesman Le Hai Binh said in the statement.

He added that Cambodia should take "appropriate measures" to deal with the activists and not let the incursion happen again.

Videos posted on social media showed a crowd of Cambodian activists waving flags, shouting and trying to push past uniformed Vietnamese border guards.

Cambodia expressed regret at the incident, according to a statement from the Svay Rieng provincial administration, and said ten Cambodians had also been injured during the scuffle.

The Cambodian authorities had not been informed in advance of the border visit and said the activists had "aimed to cause insecurity along the border".

Opposition CNRP leader Sam Rainsy said the activists were "faultless" and called on authorities to take stronger measures against "foreigners who look down on Cambodians... and swallow the(ir) land".

Opposition lawmakers, including Real Camerin who was at the border during the incident, have said the Vietnamese authorities attacked them first.

Vietnamese troops invaded to oust Cambodia's genocidal Khmer Rouge regime in 1979 and stayed in the country for a decade before withdrawing in 1989.

Ties between Hanoi and Phnom Penh have remained friendly under the administration of Prime Minister Hun Sen, a strongman who rose to power during the Vietnamese occupation and has run Cambodia for the last 30 years.