Vietnamese leader Nguyen Phu Trong died on Friday aged 80 "after a period of illness," the ruling Communist Party said.

The announcement came a day after the party that Trong led said he would hand over power to focus on receiving medical treatment for an undisclosed medical condition.

On Friday, state media announced Trong had passed away in the afternoon at a military hospital "due to old age and serious illness."

"There will be a special statement on the organisation of the funeral at the national level," state media added.

Trong's responsibilities have been handed to the country's largely ceremonial president, To Lam, a powerful former public security minister long seen as jockeying for the top job.

"The Politburo calls on the entire party, people and army to have absolute trust in the party's leadership and state management," it said on Thursday.

The country's communist regime, which is in the midst of a complete overhaul, has undergone a series of upheavals in recent months, with ministers, business leaders and two presidents all falling from grace as part of a vast anti-corruption campaign.

Lam was voted in as president in May by Vietnam's rubber-stamp parliament after his predecessor was forced to resign as part of the anti-graft drive.

Analysts said at the time that Lam, who was deputy head of the steering committee on anti-corruption matters, had weaponised investigations to take down his political rivals.

Headed by the party general secretary, Vietnam's leadership structure gives the president the second-most authority, and also includes the prime minister and the head of the National Assembly.

Trong's poor health had fuelled widespread speculation that he would not be able to stay in power until the 2026 party congress, which is expected to appoint a successor.

In power since 2011, Trong enjoyed remarkable longevity in office, during a mandate that rights groups say has coincided with increasing authoritarianism.

Known for being a technocrat and on good terms with Beijing, he structured the party around himself, benefiting from a decade of economic growth that strengthened his legitimacy. (AFP)