South Korea's opposition New Democratic Party outpolled the ruling Democratic Republican Party in yesterday's parliamentary elections, dealing a sharp setback to the government of President Park Chung Hee.

Election Officials reported today that in the contest for 154 of the National Assembly's 231 seats, the opposition party won 34 percent of the popular vote 32 percent for the ruling party.

Although final returns will not be in until Thursday, this marks the first time that South Korea's ruling party has recieved less popular support than a major opposition party in a parliamentary election.

The ruling party won 68 seats to the 61 for the New Democrats. Three seats went to the Democratic Unification Party, a splinter group, and 22 went to independent candidates, including two in jail for alleged violation of the election law.

Neverltheless, Park's government is sure to retain control of the assembly. Seventy-seven seats will be filled later this month through elections by the National Conference for Unification, an electoral College that, According to the constitution adopted six years ago, must choose from a list of candidates hand picked by Park.

The opposition New Democrats campaigned for the restoration of more democratic freedoms and the repeal of a three-year-old presidential emergency decree banning all forms of antigovernment protest. The trend against Park was particularly strong in the major cities of Seoul and Pusan.

The rise in opposition support comes as a blow to the Park government in following waning U.S. Congressional support, the beginning of the U.S. troop withdrawal from South Korea, and reported increases in domestic dissident activity.

Although President Carter has not singled out South Korea for criticism on human rights, U.S. government sources recently indicated that the Carter administration expects "some action" to be takne to improve human rights before Carter meets with Park next year.

Meanwhile, 219 American combat troops left South Korea for Fort Riley, Kan., today as the first step in President Carterhs plan to withdraw all American ground troops from Korea by 1982.

The Members of the 2nd Battlaion, 9th Infantry, 2nd Division, left Osan Airbase, 30 miles south of Seoul to a sendoff from a military band. Park, who opposes the troop withdrawal, sent no representatives to the ceremony. Another 351 GIs are to leave South Korea on Friday.

More troops, mostly elements of combat support units, are scheduled to leave by the end of this year, fulfilling Carter's plan to bring home a total of 3,400 troops in 1978. By 1979, 6,000 will have left.

No firm schedule has been set beyond 1979 for further troop withdrawals.

When Carter announced the troop withdrawal plan in 1977, many South Korean government leaders and U.S. military officials expressed concern about the possible impact it might have on South Korea's security.

The Seoul government, however, Eventually accepted the plan and an American promise to strengthen the combat capability of the 600,000-man South Korean army.