NEW YORK (AP) _ American Airlines cut fares by 75 percent Friday for all mainland U.S. flights from Jan. 8 to Feb. 10, heightening a struggle to capture a greater number of bargain travelers from low-cost competitors.

The announcement was the latest extension to fare-cutting that already has blanketed the Christmas and New Year holidays. Republic Airlines and no-frills carrier People Express also have post-New Year sales.

Northwest Airlines said it would match American's discounts in markets where they compete. Other major airlines said they were studying the American ''Winter Wonder Fare Sale,'' and spokesmen for United Airlines and Delta Air Lines said they intend to ''remain competitive'' with American.

Thomas Plaskett, American's vice president of marketing, said prices are based on mileage, ranging from $29 one way for destinations under 100 miles, to $129 for routes exceeding 2,000 miles.

Passengers must buy tickets within three days of making a reservation and stay over at least through one Saturday night, he said, and cancelations will cost 50 percent of the purchase price.

''Certainly the growth of low-cost carriers has an impact on us, as it does all carriers,'' Plaskett told a telephone news conference. ''One of our objectives is to capture an appropriate share of the price-sensitiv e market.''

He said 40 percent of American's capacity would be available for the low- cost tickets, which cost about 25 percent lower than the carrier's ''Ultimate Super-Saver'' fare, inaugurated a year ago.

Plaskett declined to speculate on whether American's pricing policy could provoke a rapidly escalating fare war, but said, ''these prices are awfully, awfully low and all of us have to make a buck.''

Wall Street analysts interpreted American's strategy as a selective, carefully conceived discount in a market that has become increasingly competitive because of low-cost carriers. American is the second-largest U.S. airline after United and historically has been regarded as a pricing leader.

''This is not a 'let's have a fare war' response,'' said Timothy P. Pettee, an analyst with L.F. Rothschild, Unterberg, Towbin, a New York investment firm. ''This is a very rational approach. It's not a People Express type of move.''

Russell Marchetta, a spokesman for People Express, said American's discounts are still more expensive in many markets, including the coast-to- coast routes, where People is charging $99 one way.

''We see this as competition, but not a threat,'' Marchetta said. ''We still have lower prices.''

Commenting on American's restrictions in the sale, he said, ''We don't tell somebody they have to stay over Saturday, and 100 percent of our seats are discounted.''

Yet, people has no guaranteed seating, handles baggage for a fee and charges for food.