Muhammad Ali: 'Greatest' boxer, showman, ambassador
There will never be another Muhammad Ali, the world reacts
"RIP Muhammad Ali. You were an exemplary sportsperson & source of inspiration who demonstrated the power of human spirit & determination," Modi, who is in Afghanistan, wrote on his Twitter page.
The funeral arrangements for Ali would be announced later.
A three-time world heavyweight champion and an Olympic gold-medallist, Ali was widely revered for his active role in fighting for civil rights.
Ali had been hospitalised multiple times in recent years. He spent time in hospital in 2014 after suffering a mild case of pneumonia and again in 2015 for a urinary tract infection.
There will never be another Muhammad Ali, the world reacts
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'The greatest, the boldest, the prettiest!'