The world culture body UNESCO on Friday confirmed Hanoi's ancient Imperial Citadel as a World Heritage site during a ceremony to begin celebrations marking the city's 1,000th anniversary.
Before a statue of King Ly Thai To, who established the capital in 1010, UNESCO's director general Irina Bokova handed over to Hanoi's mayor a large certificate that confirmed the special status of the citadel.
UNESCO had announced in August that the site and 14 others around the world would join its heritage list.
The remains of the ancient citadel and relics from feudal dynasties were first discovered about eight years ago during excavation work to build a new national assembly.
Archeologists found thousands of artefacts, including terracotta figures of dragons and phoenix heads, ceramics, cannon, swords and coins.
The dig unearthed ancient palace foundations and the remains of the central Forbidden City, with ruins dating back 1,300 years to the Chinese Tang dynasty.
World Heritage status "is an honour that creates new commitment and responsibilities for you all", Bokova told invited guests at the ceremony near Hoan Kiem Lake, the centre of the 10-day millennium celebrations.
"You owe it to humanity to promote, protect and transmit this heritage to future generations, to young people, so that they in turn may tell their children the story of King Ly Thai To", Bokova said.
The king moved the capital of Vietnam from Ninh Binh to Hanoi on the Red River Delta and called it Thang Long, or "soaring dragon", symbolising the desire for independence after a millennium of Chinese domination.
UNESCO says the citadel was the centre of regional political power for almost 13 centuries and the site reflects "a unique Southeast Asian culture specific to the lower Red River Valley," at the crossroads of influences from China in the north and the Kingdom of Champa to the south."
Work began in January on a project to conserve the citadel. Japan is providing more than 1.1 million dollars for the three-year project, a local government official said at the time.