The Federal Reserve has ordered an emergency interest rate cut of a half a percentage point to cope with the worst financial crisis since the 1929 stock market crash.

Fed Chairman Ben Bernanke and his colleagues ratcheted down their key rate by 0.5 percent, to 1.5 percent. The action revives the central bank’s rate-cutting campaign which had been halted in June out of concerns that those low rates would worsen inflation. Since then, however, economic and financial conditions have dangerously deterioriated, forcing the Fed to reverse course.

The fact that the Fed felt it could not wait until its regularly scheduled meeting late this month underscored the urgency of the situation.