NEW YORK (AP) _ For six hours, technicians failed to react to alarms warning of trouble that led to a telephone failure and air traffic control disruption that delayed hundreds of flights nationwide, AT&T said Wednesday.

AT&T blamed a power disruption at a phone switching center in Manhattan for the problem, which virtually closed three large airports for several hours Tuesday night.

Consolidated Edison, the local electric utility, asked AT&T to use internally generated power at the station because Tuesday's 93 degree heat created heavy energy demand.

Devices that are supposed to change the station's power from AC to DC failed, and the station began operating on battery power - setting off separate audio and visual alarms, said Joseph Nacchio, an AT&T vice president.

The alarms went off for six hours. Visual alarm signals at a control panel apparently went unnoticed and audio alarms within the plant drew no attention because no one went into the area where they are, Nacchio said.

''If anyone heard them and whether anyone responded to them, that's what we're investigating,'' he said.

Circuits that carry flight data between airport computers didn't work, and controllers were forced to describe the flight plans by telephone, which took much longer, said Federal Aviation Administration spokesman Duncan Pardue.

Critics asked why there was no backup in place for the air traffic control system. AT&T said a backup under construction would be ready within a month.

''A single point failure should not be capable of bringing down the majority of the FAA's communication capabilities,'' said John Mazor, a spokesman for the Air Line Pilots Association.

More than 200 flights were disrupted at John F. Kennedy and LaGuardia airports in New York and at Newark International Airport in New Jersey, delaying thousands of passengers. Most backlogged flights were cleared by Wednesday morning.

The power failure also cut off half of American Telephone & Telegraph Co.'s long-distance phone traffic into and out of New York City.

The effects rippled through the country. Airport officials in Philadelphia, Boston, Chicago, Los Angeles, Atlanta and Minneapolis-St. Paul reported delays.

The trouble began when AT&T switched to internal power at the switching station, which directs phone traffic from the local phone company to AT&T's long-distance network.

Power rectifiers, devices that are supposed to change AT&T's power from AC to DC failed, sending systems onto backup battery power, which ran out after six hours.

Technicians should have responded to alarms and the supervisor should have ordered an inspection when they went to internal power, Nacchio said. The standard plant inspection would have detected the audio alarms.

''If you don't run the operation right, you're gonna have a problem,'' he said.

When AT&T tried to shift back to commercial power, engineers discovered the power rectifiers weren't working, the company said. Twenty minutes later, before the problem could be fixed, battery power ran out and the system crashed.

If a telephone circuit fails, FAA computers are supposed to switch data to another route. But Tuesday's failure made rerouting impossible, said FAA spokesman Fred Farrar.

FAA officials planned to meet with AT&T to discuss ways to prevent a recurrence, Farrar said.

Tony Dresden, a spokesman for the National Air Traffic Controllers Association, said main and backup systems should be run on separate power sources to prevent such problems.

''The FAA is putting all their eggs in one basket and the basket tipped over,'' Dresden said.

Flight delays also resulted in January 1991 when an AT&T fiberoptic cable connecting New York and Newark was accidentally cut.

Berge Ayvazbian, who heads the telecommunications research unit of Yankee Group Inc., a high-tech consulting firm, blamed AT&T for providing a system that left the FAA vulnerable.

''It's their responsibility to make their customers fully aware of the restoration capabilities they have to offer,'' he said.

''It's an AT&T responsibility,'' the FAA's Pardue said. ''They have the capability of transferring the calls to other lines and they are responsible for delivering it. I think everyone would agree that we need a backup system. There wasn't one in place.''

Herb Linnen, an AT&T spokesman, said a facility is being built in Ronkonkoma, Long Island, to provide other circuits for the FAA. He said the facility would be open by next month.

''A month from now, the FAA wouldn't have a problem,'' Nacchio said.

The FAA has had a separate communications system in the works for some time, Pardue said. Calls and data would be transmitted through microwave dishes, avoiding the use of cables.

The system could become operational as early as 1992, Pardue said.