NEW YORK (Reuters) - A Republican prosecutor declared victory on Tuesday night in a special election to replace disgraced U.S. Representative Michael Grimm, a month before his sentencing for federal tax evasion.

Unofficial New York City Board of Elections results showed Staten Island District Attorney Dan Donovan ahead of Democrat Vincent Gentile by about 8,000 votes, holding a lead of 59 percent to 39 percent.

"I want to thank you, the voters, for placing your faith in me to serve as your Congressman!" Donovan said in a statement on his Facebook account late on Tuesday. "I truly believe our nation is at a crossroads, but with that comes great opportunity."

Gentile, a member of the New York City Council and former state senator, could not be immediately reached for comment.

Donovan drew national attention last year for presenting evidence to a grand jury that did not indict a white police officer in the choking death of an unarmed black man, Eric Garner, during an attempted arrest.

The district, which encompasses the borough of Staten Island and part of Brooklyn, is New York City's only Republican-leaning congressional district.

Grimm, a Republican elected to a third term while under federal indictment, resigned on Jan. 5. He pleaded guilty to tax evasion charges related to a Manhattan health food restaurant he partly owned, Healthalicious, and faces up to three years in prison at his June 8 sentencing.

A former FBI agent once considered a rising political star, Grimm was caught on camera last year threatening a reporter questioning him about an investigation into his campaign funds.

He told NY1 television reporter Michael Scotto: "I'll break you in half. Like a boy."

(Reporting by Barbara Goldberg and Dan Whitcomb; Editing by Bill Trott, Peter Cooney and Alex Richardson)