The king wielded great influence among millions of Zulus and beyond through his largely ceremonial and spiritual role, despite having no official power in modern South Africa.

In a short statement, the palace said the king "took a turn for the worse and he subsequently passed away in the early hours of this morning".

Born in Nongoma, a small town in the south-eastern Kwa-Zulu Natal province, Zwelithini ascended the throne in 1971 during the apartheid era. No succession plans have been divulged yet.

His body will be taken from a Durban hospital to the palace in Nongoma, around 300 kilometres away, to lie in state for "a couple of days", said Mangosuthu Buthelezi, a powerful veteran politician who is also a Zulu prince. 

President Cyril Ramaphosa has granted the king a special official funeral of the type usually reserved for presidents and ex-presidents.

The date is yet to be announced, but in the meantime national flags will fly at half-mast.

During a half-century-long reign, Zwelithini revived festivals celebrating virgin women, was blamed for fuelling deadly xenophobic violence, slammed gays as "rotten" and enjoyed a lavish and hedonistic lifestyle in a country where millions live in poverty.

Nevertheless, Ramaphosa said the king "will be remembered as a much-loved, visionary monarch who made an important contribution to cultural identity, national unity and economic development".

Culture of defiance

Returning from hiding over assassination fears, Zwelithini was crowned the eighth Zulu monarch at the age of 23.

Under the white-minority regime which ended in 1994, kings ruled homelands where most blacks were confined to defuse broader national struggles.

He was to become the longest-reigning of all known Zulu kings - dying during his 50th year on the throne.

The Zulus are South Africa's largest ethnic group with over 11 million people.

He was "very popular because he came from a very powerful and defiant line of kings," one of whom defeated the British during the 1800s Anglo-Zulu war, said historian and cultural analyst Ntuli Pikita.

And Zwelithini maintained that culture of defiance.

Traditional rulers play a largely symbolic role in modern South Africa, where they are constitutionally recognised. 

"We have lost one of the giants of the institution of traditional leadership on the continent,"  Zolani Mkiva, secretary general of a pressure group, the Congress of Traditional Leaders of South Africa, told AFP.

His home province has declared a week of mourning, with its leader Sihle Zikalala describing him as "the father and pillar of our nation... our unique torch-bearer during some of the darkest times that this country has ever seen".

Courting controversy

In 2015, Zwelithini gained international notoriety for anti-foreigner remarks suggesting immigrants were responsible for rising lawlessness in South Africa, blamed for inflaming a spate of xenophobic attacks on mostly African migrants.

Zwelithini later denied whipping up xenophobic sentiments, saying his remarks were taken out of context.

"If it was true I said people must kill each other, the whole country would (have been) reduced to ashes," he said.

A descendent of the all-powerful Shaka -- who ruled the Zulu nation until his assassination in 1828 -- Zwelithini revived the annual Reed Dance in 1984, where thousands of bare-breasted young women celebrate their virginity by dancing in front of the king.

He was the most prominent among a handful of traditional rulers who hold sway over emotive issues such as land ownership in South Africa.

In 2018, he sought an exemption for nearly three million hectares of royal land which the government had wanted to expropriate for redistribution to landless marginalised blacks sidelined by apartheid.

"All hell will break loose" if his ownership was challenged, Zwelithini warned.

He also sparked a storm in 2012 when he slammed same-sex relationships as "rotten", drawing rebuke from rights groups.

But he was lauded for his dispelling myths around HIV-AIDS in a country which has the biggest HIV epidemic in the world.

In 1994, he sparked fears of a secessionist conflict when he rallied up to 50,000 stick-wielding men -- most of them supporters of the Zulu nationalist Inkatha Freedom Party (IFP) -– to march through Johannesburg to support his call for sovereignty ahead of the country's first democratic election.

The marchers engaged in a firefight outside the headquarters of the IFP's main rival, the now-ruling African National Congress, leaving 42 people dead.

Zwelithini enjoyed the trappings of his royal status, receiving some 60 million rand ($4 million) in yearly allowances from government to help fund a lifestyle that includes several royal palaces, six wives and over 28 children.