After decades of attempts by various professional leagues to Americanize the world's most popular sport, Major League Soccer announced yesterday that starting next season it would revert to virtually all the rules used throughout the world. The league also took other steps to try to attract American soccer fans, including the introduction of regular Saturday and Monday telecasts.

The most notable changes by the four-year-old league are the elimination of the shootout and the shortening of the season by almost two months. Other major changes involve the realignment of the 12 teams to 3 divisions from 2, and the referee's designation as the official timekeeper. Without the shootout and with the referee keeping time while the game clock goes from zero up instead of the other way around, the league will play the game the way it is played in the more than 160 countries where soccer is the top sport.

M.L.S. Commissioner Don Garber said yesterday that the changes were made to comply with the wishes of the some 60 million people in the United States who considered themselves soccer fans.

''We have to go back and shore up our existence with the core soccer fan,'' Garber said in a conference call in which he announced next season's changes. ''We found through research that millions of people played soccer without a shootout. We want to build the model with core fans, instead of going over it.''