NEW YORK - Wall Street extended its 2008 plunge Thursday, with the Dow tumbling more than 300 points, giving the blue-chip average its lowest close since March and its worst three-day percentage decline since October 2002.

Thursday's decline came after a regional Federal Reserve report showed a sharp decline in manufacturing activity and as investors feared that downgrades of key bond insurers could trigger further trouble with souring debt.

Rating agency Moody's Investor's Service placed bond insurer Ambac Assurance, whose parent company is Ambac Financial, on review for a possible downgrade. That possibility alarmed investors because it would place all bonds insured by Ambac on review, and Wall Street is concerned that bond insurers would be unable to absorb a spike in claims.

The tech-heavy Nasdaq dropped 47.69, or 2 percent, to 2,346.90, giving it a 2008 deficit of 11.5 percent. Among the 150 largest companies in Silicon Valley, share prices fell a median 2.2 percent Thursday and are down a median 12.9 percent so far this year.

Among valley companies, Cisco Systems closed at $24.33, down 3.3 percent; Hewlett-Packard closed at $43, down 3 percent; Google closed at $600.79, down 2.5 percent; and Intel closed at $19.33, down 2.8 percent.

The Dow opened higher and at one point was up more than 50 points. But by the end of the day, it was down 306.95, or 2.5 percent - its lowest close since March 16, 2007. The Dow, which closed at 12,159.21, has not closed below 12,000 since November 2006.

Though there have been just 12 trading days so far in 2008, the Dow's frequent triple-digit losses have now forced it to give back all its 2007 gains. For the year, the Dow is now off 8.3 percent.

The broader market indicators also plummeted. The S&P 500 index lost 39.95, or 2.9 percent, closing at 1,333.25, and leaving it was a year-to-date loss of 9.2 percent.

Thursday's close was the lowest for the S&P 500 since October 2006 and the worst for the Nasdaq since March 2007. Like the Dow, both indexes have forfeited all of their 2007 gains. For the year, the Nasdaq is down 11.5 percent.

Declining issues outnumbered advancers by more than 5 to 1 on the New York Stock Exchange, where volume came to a heavy 2.17 billion shares compared with Wednesday's 2.11 billion.

Bond prices rose as stocks fell and anxious investors sought the safety of government-issued securities. The yield on the benchmark 10-year Treasury note, which moves opposite its price, fell to 3.63 percent from 3.74 percent Wednesday. The dollar was mixed against other major currencies.

The Chicago Board Options Exchange's volatility index, known as the VIX, and often referred to as the "fear index," jumped nearly 17 percent Thursday.

The manufacturing reading from the Philadelphia Federal Reserve caught Wall Street by surprise - igniting fears that the economy is slowing precipitously and that policy-makers might be too late in contemplating aid.

Economists had expected the Philadelphia index to come in at a negative 1.5, according to Dow Jones Newswires. Instead, the negative 20.9 figure was the weakest since October 2001, when the economy was reeling from the shock of the Sept. 11 terror attacks.

Other economic reports added to investors' glum mood. The Commerce Department said housing starts plunged 14 percent to 1.01 million in December, marking the weakest pace of home building in more than 16 years. In addition, permits to build new homes dropped 8 percent last month to 1.07 million, the lowest level since 1993.

The week's steady flow of news, much of which has dented investor sentiment, has led to a growing chorus of calls for the Fed to cut rates. The Fed's monetary policy committee meets Jan. 29 and 30 and is widely expected to lower its federal funds target from the current 4.25 percent level. Federal Reserve Chairman Ben Bernanke on Thursday reiterated recent signals that the central bank will reduce rates for a fourth straight time.

The economic concerns come in a week in which some of Wall Street's biggest names have posted huge losses following bad bets on mortgage investments.

Merrill Lynch on Thursday posted a massive loss that underscored the depth of the economy's credit problems. Morgan Stanley and Bear Stearns also have posted losses for the last three months of 2007.

Moody's announcement that it will review Ambac came after the insurer booked a $5.4 billion write-down on its credit derivative portfolio during the fourth quarter.

Ambac plunged $6.73, or 52 percent, to $6.24.