The general secretary of Vietnam's Communist Party Nguyen Phu Trong -- considered the country's top leader -- died on Friday aged 80, his party said.

In a statement, it said Trong, who had led the party since 2011, died "due to old age and serious illness" at a military hospital in Hanoi.

The announcement came a day after the party announced Trong would hand the reins of power to the country's president and former public security minister To Lam, long seen as jockeying for the top job.

At the time, the party said Trong would be focusing on treatment for an undisclosed medical condition, the first time it had referenced longstanding speculation about the ageing leader's health.

There were no further details about Trong's illness on Friday, and the party said it would later make "a special statement on the organisation of the funeral at the national level".

Trong is the first party general secretary to die in office since the death in 1986 of Le Duan, a brother-in-arms of Ho Chi Minh.

He is also the first leader to have held three consecutive mandates at the head of the party, after the liberalisation of the economy in 1986.

US President Joe Biden called Trong a "champion of the deep ties" between the American and Vietnamese people, and said both countries were more secure because of the bilateral friendship fostered by the late leader.

Russian President Vladimir Putin hailed Trong as a "true friend" of Russia.

China's communist party sent a message of condolences to its Vietnamese counterpart, while North Korean leader Kim Jong Un sent a similar message, according to state media in those countries.

Vietnam'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. 

On Thursday, when it was announced Trong was handing over his duties, the politburo called "on the entire party, people and army to have absolute trust in the party's leadership and state management".

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.

'Astonishing efficiency'

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. 

He enjoyed remarkable longevity in office, during a mandate that rights groups say has coincided with increasing authoritarianism.

Nguyen Phu Trong in 2021. Photo: AFP

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. 

"He restructured and reorganised the party around him" through his anti-corruption drive, said Benoit de Treglode, research director at the Institute for Strategic Research at France's military academy in Paris.

"Since 2011, he has cleaned up with astonishing efficiency."

The anti-corruption drive, which analysts say is also linked to political infighting, has swept through the party, police, armed forces and business community.

More than 3,500 people have been indicted since 2021, official figures show, while those sent to prison include a former health minister and two previous mayors of Hanoi.

Following a scandal related to the COVID-19 pandemic last year, President Nguyen Xuan Phuc resigned and two deputy prime ministers were removed from their positions.

Many other prominent officials working across a wide range of sectors, from environment and energy to healthcare and banking, are under investigation.

But the campaign has had unintended consequences. With many fearful of being caught in its crosshairs, everyday transactions within the business sector and the state apparatus have slowed.

Trong would like to be remembered "as a populist, someone very close to the people, who listens to the people", said Linh Nguyen, lead analyst for Control Risks in Vietnam.