The government has become increasingly jittery about public manifestations of dissent since large protests swept through Vietnam in June.
The Hanoi People’s Committee ordered police and army units in the capital to break up big gatherings.
It called for increased security at sites where independence commemorations will be held and for close monitoring of potential protesters.
In Ho Chi Minh City, leaflets were handed out warning of possible protests. Propaganda speakers broadcast warnings for people to be wary of “hostile forces”.
The authorities have not said what the anticipated protests will be about.
Police were taken aback in June when thousands took to the streets in major cities to protests against new economic zones that many fear will be dominated by China.
Protests could also erupt over unpopular new cyber security laws, environmental pollution and contested land grabs by local authorities.
The government has attempted to portray demonstrators as stooges of shadowy foreign based groups which they denounce as terrorist organisations.
As if to highlight the threat, state media publicised the arrest last week of an alleged member of the US based opposition group, Viet Tan, which Hanoi accuses of plotting terrorist attacks.
It said that Le Quoc Binh had smuggled guns and ammunition across the border from Cambodia.
Viet Tan says that it campaigns for democratic freedoms in Vietnam and offers no support for a violent overthrow of the regime.
National Day marks the 73rd anniversary of the day in 1945 when Ho Chi Minh declared national independence following the defeat of Japan.
It was another three decades before Communist forces consolidated their grip over the whole country following wars against the French, the anti-communist government in Saigon and the United States.