WASHINGTON-Acting in an emergency conference call, the Federal Reserve moved Wednesday to bolster the flagging economy by cutting interest rates by half a point for the fourth time this year.

It marked the most aggressive credit easing during Federal Reserve Chairman Alan Greenspan’s nearly 14 years at the helm and sent a clear message that the central bank is prepared to do whatever it can to prevent a recession.

Wall Street, which had given up hope that the Fed would cut rates again before its meeting on May 15, soared on the news.

The Dow Jones industrial average enjoyed its third biggest one-day point gain in history, rising by 399.10 points, or 3.9 percent, to close at 10,615.83. The technology-heavy Nasdaq rose 8.1 percent, its fourth best percentage gain, to close at 2,079.44.

The Fed’s decision to cut the federal funds rate, the interest that banks charge each other, to 4.5 percent will translate into lower borrowing costs for millions of Americans.