Undated (AP) _ The rare phenomenon of a summer air fare war broke out Wednesday, as several major airlines matched a discount program announced a day earlier by Continental Airlines.

Houston-based Continental, often a fare-cutting leader, said Tuesday it reduced to $75 each way its weekend fares through Labor Day on most non-stops, and lowered transcontinental fares to $100 each way.

Tickets for the lowered fares must be purchased on a round-trip basis and are non-refundable.

The cuts were quickly matched Tuesday by Northwest Airlines and Alaska Airlines. American Airlines and United Airlines, two of the largest carriers, joined in Wednesday, along with Braniff Inc. and Trans World Airlines.

''For competitive reasons, we will match this,'' said Bob Blattner, a TWA spokesman in St. Louis, Mo.

Fares normally hold steady or rise in summer, when travel demand is at its peak.

''Recent research shows that more and more Americans want to take several more short vacations, or long weekend trips,'' Continental spokesman Ned Walker said Tuesday. ''So what we've done is develop this nationwide weekend fare program to meet this growing trend.''

Tickets must be purchased by Aug. 11 and must be paid for within 24 hours after a reservation is made. Travel must begin on a Thursday, Friday or Saturday.

On some of the airlines, the return flight must be on that same weekend or the Monday following it. On others, the return may be on any Saturday, Sunday or Monday through Labor Day.

About 750,000 seats will be available on Continental for the lowered fares on each of the five weekends, according to the airline.

Under the new fares, a round trip from Denver to Boston will cost $150 on Continental, compared with the carrier's current lowest, advance-purchas e MaxSaver fare of $318. A round trip from Los Angeles to New Orleans will cost $150, compared with the lowest MaxSaver fare of $298.

A New York-Seattle roundtrip on Continental will cost $200, compared with the $358 MaxSaver fare, and a Los Angeles-to-New York round trip also will cost $200, compared with the $358 MaxSaver price.