Crowds of mourners gathered in India's financial capital Mumbai on Thursday for the funeral of industrialist Ratan Tata, hailed as a "titan" who led one of the country's biggest conglomerates.

Tata, who died aged 86 on Wednesday, transformed the Tata Group into a sprawling international enterprise, with a portfolio ranging from software to sports cars.

His coffin, draped in an Indian flag, was flanked by a guard of honour, with a marching band of trumpets and drums accompanying the procession.

Mumbai has declared a day of mourning, with the funeral rites to take place on Thursday afternoon.

"A titan of Indian industry", The Hindu newspaper called him on its front page. "India loses its crown jewel", the Hindustan Times wrote.

Tributes also poured in from fellow industrialists, with Asia's richest man Mukesh Ambani saying it was a "big loss, not just to the Tata group, but to every Indian".

Tata was born in Mumbai in 1937 into a family of Parsis -- a proud but dwindling community which played an outsized role in the city's business affairs under British rule.

He had intended to chart his own course in life as an architect after graduating from Cornell University in New York.

'Visionary'

But an appeal from his grandmother saw him return to India in 1962 and join the sprawling family business, beginning work as a factory floor labourer and sleeping in a hostel for trainees.

He took over the family empire in 1991, riding the wave of the radical free-market reforms India had just unleashed that year.

Tata's 21 years at its helm saw the salt-to-steel conglomerate expand its global footprint.

His 2008 decision to purchase Britain's loss-making Jaguar and Land Rover carmakers for $2.3 billion burnished his reputation when Tata Group was able to restructure both brands and return them to profit the following year.

The Tata Group said his philanthropy work "touched the lives of millions."

"From education to healthcare, his initiatives have left a deep-rooted mark that will benefit generations to come," the company added.

Indian Prime Minister Narendra Modi called Tata "a visionary business leader, a compassionate soul and an extraordinary human being."

Modi praised Tata for providing "stable leadership to one of India's oldest and most prestigious business houses".