NEW YORK — Peter B. Madoff, who was arrested early Friday, pleaded guilty to criminal actions that enabled his brother, Bernard L. Madoff, to conduct the largest Ponzi scheme in history.

While Peter Madoff, 66, acknowledged wrongdoing, he said in the hearing at the US District Court in Manhattan that he did not know of the fraud that wiped out about $65 billion in paper wealth.

The formal charges against him included falsifying documents, filing false tax returns and lying to regulators. Prosecutors said that from 1998 to 2009, Peter Madoff, who served as the senior legal and compliance officer for his brother’s firm, received $40 million from the firm, on which he did not pay taxes.

As part of his guilty plea, Peter Madoff has agreed to a 10-year prison term.