The Aga Khan, one of the world's richest men and the spiritual leader of the world's millions of Ismaili Muslims, has died.

The 88-year-old died in Lisbon earlier today, surrounded by family, The Aga Khan Development Network's social media accounts announced.

The announcement of his designated successor will follow, the organisation said.

"Leaders and staff of the Aga Khan Development Network offer our condolences to the family of His Highness and to the Ismaili community worldwide," it said, as it paid tribute to the philanthropist.

"As we honour the legacy of our founder, Prince Karim Aga Khan, we continue to work with our partners to improve the quality of life for individuals and communities across the world, as he wished, irrespective of their affiliations or origins."

Prince Karim Al-Hussaini, Aga Khan IV, was the 49th imam of the Nizari Ismaili Shia Muslims and is thought to have had a vast fortune.

The Aga Khan, who became the spiritual leader of the Shia Ismaili Muslims at age 20 as a Harvard undergraduate, poured a material empire built on billions of dollars in tithes into building homes, hospitals and schools in developing countries.

The extent of his financial empire is hard to measure, though reports estimated his personal wealth to be in the billions.

He was passionate about horse racing, and owned one of the sport's most famous ever animals in Shergar, who won the Epsom and Irish Derbys as well as the King George, as per .

Treated as a head of state, the Aga Khan was given the title of "His Highness" by the late Queen Elizabeth II in July 1957, two weeks after his grandfather the Aga Khan III unexpectedly made him heir to the family's 1,300-year dynasty as leader of the Ismaili Muslim sect.

Prince Karim had a close relationship with the British Royal Family, and was a friend of Queen Elizabeth, who hosted him for a dinner at Windsor Castle in 2018 in celebration of his Diamond Jubilee, as per .

Prince Karim's ancestors had fled Persia for India two centuries ago. He was born in Switzerland and spent much of his later years living France, though he had British citizenship, as per .

He became the Aga Khan IV on Oct. 19, 1957, in Dar es Salaam, Tanzania, on the spot where his grandfather once had his weight equaled in diamonds in gifts from his followers.

A defender of Islamic culture and values, he was widely regarded as a builder of bridges between Muslim societies and the West despite - or perhaps because of - his reticence to become involved in politics, as per PA.

The Aga Khan Development Network, his main philanthropic organization, dealt mainly with issues of health care, housing, education and rural economic development.

A network of hospitals bearing his name are scattered in countries where health care had lacked for the poorest, including Bangladesh, Tajikistan and Afghanistan, where he spent tens of millions of dollars for development of local economies.

The Ismailis - a sect originally centered in India but which expanded to large communities in east Africa, Central and South Asia and the Middle East - consider it a duty to tithe up to 10% of their income to him as steward.

He is survived by his children Princess Zahra, Prince Rahim, Prince Hussain and Prince Aly Muhammad, as well as his brother Prince Amyn Muhammad, half-sister Princess Yasmin, and four grandchildren.