(CNN) - U.S. Rep. Michael Grimm failed to report more than $1 million in sales and wages at a Manhattan restaurant he once ran, using unreported cash to pay workers "off the books" to "evade taxes and keep more money for himself," a federal prosecutor said.

Grimm, R-New York, pleaded not guilty on Monday in federal court in Brooklyn to 20 counts, including mail fraud, wire fraud, filing false tax returns, hiring unauthorized aliens and perjury.

A former FBI agent who used to investigate fraud, Grimm wore a dark suit and was not handcuffed during the hearing.

He was released on $400,000 bond, secured by his home in Staten Island. As conditions for his release, Grimm, 44, must surrender his guns and passport.

He told reporters that he would remain in office while fighting the charges. He's up for re-election in November.

"I'm going to fight tooth and nail ... until I am fully exonerated," Grimm said.

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