On Wall Street, his name is legendary. With money he had made as a lifeguard on the beaches of Long Island, he built a trading powerhouse that had prospered for more than four decades. At age 70, he had become an influential spokesman for the traders who are the hidden gears of the marketplace.

But on Thursday morning, this consummate trader, Bernard L. Madoff, was arrested at his Manhattan home by federal agents who accused him of running a multibillion-dollar fraud scheme  perhaps the largest in Wall Street’s history.

Regulators have not yet verified the scale of the fraud. But the criminal complaint filed against Mr. Madoff on Thursday in federal court in Manhattan reports that he estimated the losses at $50 billion. “We are alleging a massive fraud  both in terms of scope and duration,” said Linda Chatman Thomsen, director of the enforcement division at the Securities and Exchange Commission. “We are moving quickly and decisively to stop the fraud and protect remaining assets for investors.”

Andrew M. Calamari, an associate director for enforcement in the S.E.C.’s regional office in New York, said the case involved “a stunning fraud that appears to be of epic proportions.”