Gautam Adani's vast Indian business empire lost billions more dollars 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 a "brazen stock manipulation and accounting fraud scheme over the course of decades".

The three-day selloff has now erased a total of about $68 billion in the conglomerate's market value, according to Bloomberg News.

Adani Total Gas and Adani Green Energy both plunged another 20 percent on Monday and saw trading halted, after suffering the same fate Friday.

Adani Transmission ended 14.91 percent lower, although the group's flagship Adani Enterprises was 4.21 percent higher.

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

On Monday the 60-year-old was eighth in the ranking, which put his wealth at $88.2 billion, down from almost $130 billion before the allegations.

He maintained his crown as Asia's richest man, although volatile trading earlier in the day saw him briefly overtaken by fellow Indian tycoon Mukesh Ambani.

- 'Madoffs of Manhattan' -

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 the researchers' 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 the response failed to stem intense selling pressure across most of Adani's listed companies, while Hindenburg accused the group of "systematically looting the nation".

The allegations came as Adani seeks to raise an ambitious $2.5 billion via a stock sale, and despite Monday's rise Adani Enterprises shares remained well below the 3,112-3,276 rupees price range set for the offer -- making them cheaper to buy in the market.

Investors had subscribed for only three percent of the follow-on public offer (FPO), which is due to close on Tuesday, at the end of its second day of bidding on Monday.

But Abu Dhabi-based International Holding Company (IHC) gave the group a vote of confidence, saying it would buy one-sixth of the shares on offer, paying $400 million to do so.

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."