Three policemen and a villager died Thursday in rare violent clashes with Vietnam's communist authorities over disputed land around a military-owned Hanoi airport.
Construction of the Mieu Mon facility has been a long-running sore for villagers who say it is being built on land illegally seized by the military.
Clashes erupted on Thursday before dawn when authorities attempting to erect a perimeter fence were met by residents armed with "grenades, petrol bombs and knives", the Ministry of Public Security said in a statement.
The "social disorder" led to the "deaths of three policemen and one resident", the statement said, adding other villagers were "arrested for serious violations of the law".
It was not immediately possible to confirm the toll or verify the authorities' version of events, disseminated with unusual speed in a country where secrecy and control normally trump transparency.
But a video widely circulated on Facebook by an activist at the scene appeared to show gunfire lighting up the dawn gloom around the village as several truckloads of security guards arrived.
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Human Rights Watch urged Vietnam to launch an investigation that "gets to the bottom of what happened" and to provide unfettered access to the site for impartial observers including journalists, diplomats and UN officials.
Land disputes are common in Vietnam, where powerful individuals and companies often make claims on property.
The government strictly controls freedom of expression and the right to protest but flashpoints occur.
In 2017 villagers held more than a dozen police officers and officials hostage for several days at the airport site in a standoff that gripped the tightly-controlled country.