Five electric feeder cables burned out in upper Manhattan, knocking out power Saturday to some customers and forcing utility officials to cut all service in a 40-block area of Harlem to avert more widespread outages.

On Long Island's South Fork, which includes the posh Hamptons, the Long Island Lighting Company Saturday reduced power by 5 percent in a bid to prevent blackouts in the face of high electrical demand.The move caused widespread brownouts - sustained power reductions - as high demand pushed LILCO generators to their capacity. No other power failures were reported on Long Island, but officials urged the public to curtail all but essential use of electricity.

In Manhattan, police assigned additional officers to the two-mile strip affected by the outage, running from West 110th Street to West 153rd Street, from Amsterdam Avenue to the Hudson River, while Consolidated Edison workers scrambled to locate and repair the cables.

The blackout left area residents without lights, air conditioning and elevator service as temperatures climbed toward the 90s. Storekeepers along Broadway could only stand by and watch as perishable food began to rot in the midday heat.

Repair crews started cutting underground cables about 6:15 a.m. to shut off all power to the neighborhood to forestall major outages to the east and to protect overloaded equipment, Con Ed spokeswoman Martha Liipfert said.

"We are going down into manholes and cutting secondary power cables at about 80 locations along Amsterdam Avenue," Liipfert said. "This will increase the number of outages but will not increase the size of the affected area."

Arrangements were made to move customers who rely on electrically powered medical equipment, she said.

Officials warned that the area might not get electricity back until Sunday.

Four feeder cables - each about 3 inches in diameter and carrying 13,000 volts of power to 5,000 homes - short-circuited about 11 p.m. Friday and a fifth was out of action by 4 a.m., causing scattered outages.

The city Bureau of Traffic Operations said the few traffic signals still operating early Saturday no longer were synchronized, leading to early morning backups at many intersections.

The outage also caused scattered subway delays.