Pan American World Airways played the spoiler yesterday in the industry's fare war.

Two weeks after the "big three" transcontinental airlines announced they were abandoning their $108 one-way, no-strings "Supercoach fares between New York and Los Angeles later this month, Pan Am yesterday dropped its regular coach fare to that level.

The $108 one-way, cross-country fare to LA and a $125 one-way fare to San Francisco were introduced in June by United Airlines, American Airlines and Trans World Airlines after World Airways came into the cross-country scheduled air marked with regular fares that were half what the big three were charging.

But World has not been operating its cross-country routes since mid-June, first because of the DC10 grounding and now because of a strike. And all three of World's major competitors moved both to increase fares -- up to $139 to LA and $149 to San Francisco -- and to impose seven-day advance purchase requirements, limit the number of seats sold, and, in some cases, impose a minimum stay requirement.

Pan Am said yesterday that its regular coach fare -- for a reserved seat purchased anytime -- will be $108 between New York and Los Angeles and $125 between New York and San Francisco: no advance-purchase necessary, no minimum or maximum stays.

Although Pan Am has been offering an $89 one-way standby fare on all its transcontintental flights and will continue to do so, its regular coach price reduction yesterday is significant. Its regular one-way coach fares for reserved seats were until yesterday $251 to LA and $261 to San Francisco.

"This action assures that the $108 and $125 fare levels will continue to be available since other airlines plan to cancel these fares," Pan Am said in a statement.

Pan Am's schedule, however, is limited, compared with its brethren. Pan Am operates just two roundtrips a day between New York and LA and one between New York and San Francisco.

Yesterday, spokesmen for the big three gave no indication that they would change their fare plans in response to Pan Am's move. Starting Sept. 14, however. Capitol International Airways will also offer a low-fare, cross-country service. Its initial price will be $90 each way between New York and Los Angeles; after Oct. 14, the price will rise to $108 each way.