NEW YORK, AUG. 13 -- A midday electrical power outage darkened computer screens, stalled elevators and curtailed trading in much of the financial district today, forcing both the American Stock Exchange and the New York Mercantile Exchange to close several hours early.

The outage, caused by a fire in an electrical substation, sharply reduced or halted the buying and selling of foreign currencies, Treasury bonds, oil futures and other commodities for most of the afternoon.

The New York Stock Exchange, the nation's largest, was not affected because it was outside the area struck by the blackout.

"We have no screens, and we're not trading," said William Brachfeld, chief of fixed-income trading for Daiwa Securities America Inc.

Brachfeld, like other traders, said the blackout was not expected to cause any major, long-term problems.

The outage began in the southwestern corner of Manhattan shortly before noon and gradually spread as firefighters struggled to contain the blaze at a power station near the Brooklyn Bridge. Electricity was restored in most of the affected areas by 5 p.m.

Thousands of stockbrokers, secretaries and other Wall Street employees got an unexpected half-day holiday when they were unable to return to work from lunch because the elevators in their skyscraper office buildings were halted. Many had a hard time getting home because some subway service was suspended.

Several hundred people were trapped for up to 2 1/2 hours in 40 elevators in the twin, 110-story buildings at the World Trade Center, one of the 4,150 customers affected by the outage.

"We've got no lights, no air conditioning, no elevators, and it's starting to get warm," an executive at Dean Witter Reynolds, stuck on the 70th floor of the World Trade Center, said in midafternoon.

In the World Financial Center, which also was affected, American Express Co. and Shearson Lehman Brothers Inc. evacuated their offices, and Merrill Lynch & Co. sent home nonessential personnel.

A spokesman for the Wall Street Journal, which also has its headquarters in the complex, said Tuesday's editions were likely to be "slightly smaller" than normal.

All trading halted at the American Stock Exchange at 1:13 p.m., when its lights and computers went dead. Normally, the exchange stays open until 4:15 p.m.

The New York Mercantile Exchange, where oil and other commodities are traded, closed at 1:10 p.m. and did not reopen.

Some oil traders complained that they might suffer losses because the shutdown robbed them of the ability to hedge their positions, as they normally do, late in the trading day.

The New York Commodities Exchange and New York Futures Exchange also closed early.

On the Nasdaq system, where unlisted stocks are traded over the counter, about 600 of the system's 1,500 computer terminals in southern Manhattan went dead because of the outage. Roughly half of those terminals were reconnected within an hour, according to a Nasdaq spokesman.

The blackout highlighted the financial district's vulnerability to technical failures, with one expert noting that the problem could have been much worse if the outage had completely shut down telephone services.

Phone services were only partially affected because electrical backup systems were put into operation.

"We're happy to learn that we've dodged the bullet this time," said Daniel S. Bayer, who recently wrote a report warning of the potentially disastrous impact that a telecommunications failure could have on Wall Street.

Bayer is a vice president at the New York City Partnership, a business and civic organization. His report, released in June, found that a major telecommunications shutdown in southern Manhattan could block $1 trillion a day in business transactions.