Reports from Vietnam say up to 40 people were injured when police dispersed a protest by the minority mainly Christian Montagnard people.
Minority groups have several grievances against the government
Several people were arrested at the rally in Dak Lak province.
The US-based Montagnard Foundation said the rally was the first in a series of non-violent public prayer protests.
The head of the UN refugee agency in neighbouring Cambodia, Nikola Mihajlovic, said the Montagnards had valid reason to fear for their safety.
People were still crossing into Cambodia to seek asylum, he added.
The Vietnamese government say the Montagnard minorities have been inciting violence and pressing for an independent Montagnard state in the region, known as the Central Highlands.
An outbreak of protests by Montagnards in February 2001 over land rights and repression of Protestant groups led to a government crackdown.
Week of protests
Montagnard Foundation president Kok Ksor rejected government allegations that his organisation was behind the protests.
"We have not pushed them to do anything. The government pushed
them by the way they treated them," he told AFP news agency. "This
is a non-violent demonstration."
He said the protests - in the regional capital Buon Ma Thuot - were the beginning of a week of non-violent demonstrations over Easter week against religious repression, ahead of elections on 25 April.
The Montagnards have traditionally resisted control from central government, in the past opposing French colonial rule and the US-backed South Vietnamese regime.
In recent years, they have been angered by efforts by the Communist government to turn their ancestral forests into the country's largest coffee-growing region and bring in lowland Vietnamese.