(Corrects sixth paragraph to show maximum sentence in 20 years, not five years)

By Edith Honan

The former chairman of the Nasdaq Stock Market who remains a member of Nasdaq OMX Group Inc's nominating committee, is best known as the founder of Bernard L. Madoff Investment Securities LLC, the closely-held market-making firm he founded in 1960.

But the alleged fraud involved a hedge fund he ran from a separate floor of the building where his brokerage is based.

Madoff told senior employees of his firm on Wednesday that "it's all just one big lie" and that it was "basically, a giant Ponzi scheme," with estimated investor losses of about $50 billion, according to a criminal complaint against him.

A Ponzi scheme is a pyramid-type swindle in which very high returns are promised to early investors, who are paid off with money put up by later investors.

Prosecutors charged Madoff, 70, with a single count of securities fraud. They said he faces up to 20 years in prison and a fine of up to $5 million.

"Madoff stated that the business was insolvent, and that it had been for years," Lev Dassin, acting United States Attorney for the Southern District of New York, said in a statement.

Authorities said that, according to a document filed by Madoff with the U.S. Securities and Exchange Commission (SEC) on January 7, 2008, Madoff's investment advisory business served between 11 and 25 clients and had a total of about $17.1 billion in assets under management.

'UNFORTUNATE SET OF EVENTS'

"Bernard Madoff is a longstanding leader in the financial services industry," his lawyer Dan Horwitz told reporters outside a downtown Manhattan courtroom where he was arraigned. "We will fight to get through this unfortunate set of events."

A shaken Madoff stared at the ground as reporters peppered him with questions. He was released after posting a $10 million bond secured by his Manhattan apartment.

The SEC filed separate civil charges.

"Our complaint alleges a stunning fraud -- both in terms of scope and duration," said Scott Friestad, the SEC's deputy enforcer. "We are moving quickly and decisively to stop the scheme and protect the remaining assets for investors."

The SEC said it appeared that virtually all of the assets of his hedge fund business were missing.

Madoff had long kept the financial statements for his hedge fund business under "lock and key," according to prosecutors, and was "cryptic" about the firm.

Bernard L. Madoff Investment Securities has more than $700 million in capital, according to its website. It is a market maker for about 350 Nasdaq stocks, including Apple, EBay and Dell, according to the website.

The website also states that Madoff himself has "a personal interest in maintaining the unblemished record of value, fair-dealing, and high ethical standards that has always been the firm's hallmark."

(Additional reporting by Christian Plumb, Phil Wahba, Michelle Nichols in New York and Rachelle Younglai in Washington; Editing by Andre Grenon)