Zambia's former President Edgar Lungu has died at the age of 68, his party has said in a statement.

He had "been receiving specialized treatment in South Africa" for an undisclosed illness, the Patriotic Front (PF) added.

Lungu led Zambia for six years from 2015, losing the 2021 election to the current President Hakainde Hichilema by a large margin.

After that defeat he stepped back from politics but later returned to the fray. He had ambitions to vie for the presidency again but at the end of last year the Constitutional Court barred him from running, ruling that he had already served the maximum two terms allowed by law.

Even after being disqualified from running for the presidency again, he remained hugely influential in Zambian politics and did not hold back in his criticism of his successor.

In a short video, Lungu's daughter Tasila said that the former head of state, who had been "under medical supervision in recent weeks", died at a clinic in South Africa's capital, Pretoria, at 06:00 (04:00 GMT) on Thursday.

"In this moment of grief, we invoke the spirit of 'One Zambia, One Nation' - the timeless creed that guided President Lungu's service to our country," she added in an emotional statement.

There was no mention of what his condition was, but a decade ago he underwent throat surgery abroad. At the time his office said he was suffering from a narrowing of the oesophagus.

In his condolence message, President Hichilema called for "solemnity, unity and an outpouring of love and compassion.

"Let us come together as one people, above political affiliation or personal conviction, to honour the life of a man who once held the highest office in our land."

Lungu first became president in January 2015 after winning a special presidential election triggered by the death in office of Michael Sata.

After completing Sata's term, he won a further five years in power in 2016 taking just over 50% of the vote.

But after six years at the helm, Lungu, who encouraged Chinese investment and enlisted the country's help in infrastructure development, was blamed for a struggling economy, high unemployment and rising debt levels.

His time in office was also marred by corruption scandals involving his allies and relatives. Lungu always denied wrongdoing.

His party's youth wing was accused of harassing opposition supporters, and the population at large.

Lungu lost in 2021 by close to a million votes with Hichilema, seen as more pro-Western, tapping into widespread dissatisfaction among the electorate.

He said he was retiring in the aftermath of the vote, but returned to frontline politics in 2023 as his successor's popularity waned.

"I am ready to fight from the front, not from the rear, in defence of democracy. Those who are ready for this fight, please come along with me, I am ready for anything," Mr Lungu told supporters at the time.

After returning to politics, the former president complained of police harassment. At one point last year he said he was "virtually under house arrest".

"I cannot move out of my house without being accosted and challenged by the police and driving me back home", Lungu told the BBC's Newsday programme.

In the interview in May 2024, he alleged that he had been barred from attending a conference abroad and from travelling for medical treatment.

In 2023, the police warned him against jogging in public, describing his weekly workouts as "political activism".

The government said that Lungu had "never been placed under house arrest" and that he was free to exercise his rights.

Lungu was a lawyer by training but enjoyed a meteoric rise in politics after winning a seat in parliament as a PF MP in 2011.

He entered government as deputy minister in the vice-president's office in that year and rose to become minister of home affairs in just over 12 months.

He later became minister of defence and then justice. A close friend described Lungu as a "good foot-soldier, lawyer and politician, father, husband and grandparent".

Born on 11 November 1956, Lungu graduated with a law degree from the University of Zambia in 1981. He also underwent military training at the then Miltez army college in Kabwe.

He later worked at Andre Masiye and Company Advocates, Barclays Bank and Zambia Consolidated Copper Mines.

Additional reporting by BBC Monitoring