U.S. Federal Reserve Vice Chairman Roger Ferguson announced on Wednesday he will step down effective April 28, in a surprise announcement that comes at a time of upheaval at the central bank.

"My service on the board has been rewarding and stimulating, and it is now time for me to pursue other professional opportunities," he said in a letter to President George W. Bush dated February 22.

The Fed, in a separate statement, said Ferguson will not attend the March 27-28 meeting of the policy-setting Federal Open Market Committee.

Ferguson's term on the Fed board does not expire until January 31, 2014. He leaves before the new Fed chief Ben Bernanke, chairs his first policy meeting.

The last Democrat on the Fed's board, Ferguson joined the central bank in November 1997. First nominated for an unexpired term on the Fed board by President Bill Clinton, the Harvard University-trained lawyer and PhD economist was elevated to the second highest post in October 1999.

Ferguson played a key role in coordinating the Fed's response to the attacks of September 11, 2001, which happened while Alan Greenspan, who was Fed chairman at that time, was out of the country. He is the highest-ranking African American to serve on the Fed.

Before joining the Fed, Ferguson was a partner at McKinsey & Co., an international management consulting firm.