Gautam Adani's vast Indian business empire lost billions of dollars more in value on Monday after its rejection of claims of widespread fraud failed to reassure investors.

The slump was sparked by a report by US investment group Hindenburg Research that last week alleged "brazen stock manipulation and accounting fraud scheme over the course of decades".

The three-day selloff has now erased about $71 billion in market value, according to Bloomberg News. Last week Adani's firms recorded a slump of around $50 billion.

The fire sale also saw the school- dropout-turned-tycoon, who is seen as close to Prime Minister Narendra Modi, tumble from third on the Forbes real-time rich list.

On Monday the 60-year-old was at ninth position in the ranking, which put his wealth at $85.2 billion.

This meant he kept his crown as Asia's richest man, although earlier in the day he was briefly overtaken by fellow Indian tycoon Mukesh Ambani.

Adani's conglomerate said it was the victim of a "maliciously mischievous" reputational attack and on Sunday issued a 413-page statement that it said rebutted Hindenburg's claims.

Dubbing Hindenburg the "Madoffs of Manhattan" -- a reference to crooked financier Bernie Madoff -- the statement said Hindenburg's allegations were "nothing but a lie".

"This is not merely an unwarranted attack on any specific company but a calculated attack on India, the independence, integrity and quality of Indian institutions, and the growth story and ambition of India," it said.

But despite an early rebound in flagship Adani Enterprises on Monday, Adani's response failed to stem intense selling pressure across most of his listed companies.

The allegations came as Adani seeks to raise an ambitious $2.5 billion via a stock sale and have pushed the price of Adani Enterprises shares below the price band of 3,112-3,276 rupees per share.

The follow-on public offer (FPO), which was due to close on Tuesday, was subscribed only one percent at the end of its first day on Friday.

Hindenburg said in response to Adani's statement that "India's future is being held back by the Adani Group, which has draped itself in the Indian flag while systematically looting the nation".

"In terms of substance, Adani’s ‘413-page’ response only included about 30 pages focused on issues related to our report," it said.

"The remainder of the response consisted of 330 pages of court records, along with 53 pages of high-level financials, general information, and details on irrelevant corporate initiatives, such as how it encourages female entrepreneurship and the production of safe vegetables."

Shares in flagship Adani Enterprises gave up early gains to trade only 1.5 percent higher in Mumbai.

Adani Total Gas, Adani Transmission and Adani Green Energy saw trading halted again -- having suffered the same fate Friday -- after a 20-percent plunge each.

Adani Power and Adani Wilmar also hit their circuit breakers after falling five percent each.

Adani Ports turned negative after a brief rebound, to trade 1.5 percent lower.