JACKSON HOLE, Wyo.—A defiant Ben Bernanke sought to shoot down criticism of the Federal Reserve's easy-money policies and strengthen the case for new efforts by the central bank to bring down what he described as gravely high unemployment.

Markets have been on edge for months about whether the Fed will launch another large bond-buying program. Fed Chairman Bernanke, speaking Friday at the central bank's annual retreat here, offered a vigorous defense of the Fed's $2.3 trillion in bond purchases since 2008, estimating they...