Soaring temperatures have gripped parts of southern and northern India in an extreme heat wave which.
Ban called Mandela "one of the giants of the 20th century" at a New York reception for the 50th anniversary of the Organization of African Unity which played a leading international role in fighting apartheid in South Africa.
"I know our thoughts and prayers are with Nelson Mandela, his family and loved ones, all South Africans and people across the world who have been inspired by his remarkable life and example," Ban said.
"Let us all show similar conviction and sense of purpose today in working to improve well-being and opportunity for all Africans," the UN secretary general added.
Mandela, 94, is on life support in hospital in Pretoria.