Dec 6 (Reuters) - The longest-serving U.S. Attorney resigned in Louisiana on Thursday after a scandal involving prosecutors who posted derogatory comments online.

Jim Letten, the top federal prosecutor for Louisiana's Eastern District, which includes New Orleans, said in a statement the decision to step down was his. The resignation takes effect on Tuesday.

Letten's office has been rocked by a scandal involving two prosecutors who work under him, including his former top deputy. They used aliases to post defamatory comments about targets and defendants in criminal cases on a news website.

Letten was involved in the successful prosecution of former Louisiana Governor Edwin Edwards on racketeering charges. He also oversaw such cases as the deadly police shootings on a New Orleans bridge after Hurricane Katrina in 2005.

Letten was appointed by President George W. Bush in April 2001 and was the longest-serving current U.S. Attorney.

U.S. Attorney General Eric Holder said in a statement that Dana Boente, the first assistant U.S. Attorney for the Eastern District of Virginia, would be Letten's interim replacement.

Holder and New Orleans Mayor Mitch Landrieu praised Letten as a longtime crime fighter.

"He has been a great partner for the City of New Orleans as we fight public corruption and violent street crime," Landrieu said in a statement.

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