Although all four United States boxers were victorious tonight in the semifinals of the Pan American Games, their victories were dwarfed by the achievements of the Cubans they must face in Sunday's finals.

In a demonstration of power that must be unprecedented in the penultimate round of a major competition, the Cubans did not permit a single opponent to survive the first round.

Most impressive of the U.S. winners was welterweight Louis Howard of St. Louis, who pounded out a 5-0 decision over Luis Garcia of Venezuela.

The crowd, held down by rain that flooded nearby streets, tried to give Garcia some encourragement, but they were reduced to near silence after Howard floored him with a solid right during a furious flurry in the second round.

Howard will meet Jose Aguilar, who stopped Antonio Madureira of Brazil in 1:50, shortest bout of the night.

World lightweight champion Pernell Whitaker of Norfolk squeaked out a 3-2 verdict over Angel Beltres of the Dominican Republic.

Whitaker landed a steady series of body punches, with an occasional left uppercut thrown in, and his principal problem seemed to be keeping his slipping trunks up in the third round.

Nevertheless, judges from Canada and Puerto Rico voted for Beltres. The other three--from Ecuador, Peru and Jamaica--had Whitaker a winner by as much as 60-55.

Whitaker holds a 2-1 career margin over Cuban Angel Herrera, the other finalist. Herrera, like Whitaker a southpaw, stopped Alberto Cortez of Argentina in 2:02.

Light welterweight Jerry Page of Columbus, Ohio, gained a 4-1 decision over Giovanni Lopez of the Dominican Republic, who threw wild punches most of the night. Page goes up against Cuban Candelario Duverger, who twice knocked down Genaro Leon of Mexico before the bout was halted at 2:51.

The fourth U. S. winner was the Bronx magician, Dennis Milton, with a comfortable 4-1 decision over Hector Ortiz of Puerto Rico.

Milton's final foe, Cuban Oreste Solano, was not able to score a knockout. Instead, he was awarded a disqualification over Dario Mattoni of Argentina at 2:49.

Mattoni was warned for holding three times in the first 23 seconds and was disqualified when he was assessed his third one-point penalty of the round.