Vietnam held its largest-ever public celebrations on Tuesday to mark 80 years since the declaration of independence, with legions of lock-stepped patriots marching under fluttering flags.

Around 40,000 troops and civilians began parading in the capital Hanoi after dawn, feting the date when communist revolutionary Ho Chi Minh declared a "Democratic Republic of Vietnam" free from French rule in 1945.

Tanks, drones and missile batteries filed through the streets as helicopters and planes streaked above crowds which were hundreds-of-thousands strong in the sweltering morning sun.

Pham Thanh Van, a 78-year-old veteran, wore his military uniform pinned with medals earned fighting American troops as he watched from a front row seat at the Ho Chi Minh mausoleum.

"This will be my final memory. Don't forget us," he said. "I feel so proud. Independence brought development and prosperity to the country. I felt it worth fighting for."

Hanoi's top leader To Lam marked the top of the parade with a speech as China's number-three official – National People's Congress Chairman Zhao Leji – looked on, alongside influential former Cambodian prime minister Hun Sen and Cuban President Miguel Diaz-Canel.

"In this sacred moment, we respectfully remember our ancestors," Lam said.

"Our nation has overcome countless difficulties and challenges. Our country has transformed from a colony into an independent and unified nation, steadily advancing towards modernity and deep integration."

Chinese and Russian troops marched alongside their Vietnamese counterparts in the procession lasting around two hours, beginning with a squadron of helicopters trailing the national yellow-star flag and hammer-and-sickle banners over the capital.

Underneath, youngsters in traditional dress twirled giant floral tributes after artillery fired off a ceremonial salute, and an honour guard of police goose-stepped in pristine dress whites.

"It showcased Vietnam's strength," said impressed spectator 34-year-old Tran Nguyen Trung Chien. "We the people welcomed them all – this showcased Vietnam's high patriotism.

The tightly-choreographed celebrations out-scaled those staged in April to mark the 50th anniversary of the fall of Saigon, when communist North Vietnam sealed the defeat of the US-backed South.

The festivities, which state media called "unprecedented in scale", also broke the record 30,000-strong show of force that the one-party state mustered for emancipation celebrations back in 1985. (AFP)