PHNOM PENH (REUTERS) - It took just one word to fire up the mob that beat Mr Tran Van Chien to death after a minor road accident in the Cambodian capital, Phnom Penh.

The 30-year-old carpenter was standing among onlookers on Feb 16 when someone shouted "yuon", a term widely seen as derogatory to the hundreds of thousands of ethnic Vietnamese who call Cambodia home.

Seconds later, the crowd turned on Mr Tran Van Chien. "There were so many people I couldn't help," recalled his sister, Ms Tran Yaing Chang, shuffling through photos of his funeral. "He was killed instantly."

Cambodians have long borne a grudge against the Vietnamese.

For centuries Cambodia was caught between more powerful Thai and Vietnamese kingdoms and for generations, many Cambodians have believed Vietnam wants to take over their land.

At various times, Cambodian politicians have found it useful to play up that fear.

In late 1978, Vietnamese forces invaded to overthrow the Khmer Rouge who vilified Vietnam and launched cross-border raids. Vietnam occupied Cambodia for the next 11 years.

Recently, the resurgent opposition Cambodia National Rescue Party (CNRP) has stoked anti-Vietnamese sentiment, seeking to capitalise on long-ruling Prime Minister Hun Sen's association with Vietnam.

Hun Sen first came to power in a government installed by the invading Vietnamese and his enemies have long played that up in a bid to undermine his legitimacy.

The opposition tried to exploit the distrust of Vietnam in an election last year some observers called racist, with party leader Sam Rainsy routinely using the term "yuon", an old word for Vietnamese that many find offensive, to refer to Vietnamese.

Mr Kem Sokha, deputy leader of the CNRP, accused Vietnam of sending immigrants to occupy Cambodian land and promised to cancel contracts with Vietnamese companies if the CNRP won.

The CNRP has used anti-Vietnamese rhetoric since the vote last July, which Mr Hun Sen's party won amid allegations of cheating.

In a March meeting with supporters, Mr Sam Rainsy said the government planned to colonise Cambodia with "neighbours from the east" - meaning Vietnamese.

Companies from Vietnam are growing increasingly concerned.

"I'm afraid if this kind of sentiment becomes stronger, we might struggle doing business here," said a Phnom Penh-based representative of a Vietnamese company producing fertiliser in Cambodia.

"Some people, including government officials, have said Vietnamese businesses are taking advantage of Cambodians and don't bring any benefits to their country," said the executive, who declined to be identified because he is not authorised to speak to the media.

Vietnamese businesses have been targeted during anti-government rallies since the disputed election.

Vietnam's investments in Cambodia are worth US$2.5 billion (S$3.1 billion), with bilateral trade at US$3.5 billion, according to the Vietnamese Embassy in Phnom Penh. A Cambodian government survey of foreign direct investment from 1994-2011 put Vietnam sixth on a list topped by China.

A prominent human rights activist said the opposition's rhetoric was irresponsible and dangerous.

Mr Ou Virak, the former director of the Cambodian Centre for Human Rights, who has received death threats for speaking out against racism, said once stoked, anti-Vietnamese anger could be difficult to contain, and could raise tension with other ethnic groups in Cambodia such as ethnic Chinese or Cham Muslims. "All it needs is a spark," he said.

China is the country's biggest investor and aid donor, and its firms have snapped up land concessions for mining, agriculture and tourism.

Mr Ou Virak said blaming Vietnamese for evictions or the destruction of forests was a "perfect excuse" for those who did not want to upset the big-spending Chinese.

Mr Sam Rainsy declined to comment on the issue of anti-Vietnamese sentiment.

In an April 12 statement, the CNRP said accusations the party was anti-Vietnamese were "groundless", and it insisted the word "yuon" was not derogatory or racist.

About 700,000 ethnic Vietnamese live in Cambodia, said Mr Ang Chanrith, director of Minority Rights Organization, basing his estimate on interviews with Vietnamese community leaders and Cambodian authorities. Cambodia has a population of about 15 million.

Bearing the brunt of anti-Vietnamese feeling are ordinary people like Mr Tran Van Chien who lived in a ramshackle stilt house in a 400-strong community of ethnic Vietnamese - most of them very poor - on the bank of the Tonle Bassac River. Cambodians call it "Yuon Village".

His sister, Ms Tran Yaing Chang, 32, said her brother and seven other siblings had left Vietnam 10 years ago to seek a better life in Cambodia.

She owns a small shop and, until her brother's death, felt Cambodia had treated her well. Her brother was married to a Cambodian woman, who was about to deliver their first child.

"I've seen a lot of violence against the Vietnamese since the election," she said. "I dare not complain about anything, even though they treat us like this, because we are living in their country."

Many ethnic Vietnamese families have lived in Cambodia for generations, said community leader Sok Hor, 70, who moved to Cambodia in 1981.

He said many people now say verbal abuse is so common that he tells people to "stay at home and sleep" rather than go out at night.