BUENOS AIRES, Argentina (AP) _ President Raul Alfonsin's party was routed by the opposition Peronists in Sunday's national elections, setting back the president's plans to enact constitutional reforms.

Alfonsin's centrist Radical Civil Union lost its narrow majority in the House of Deputies and the governorship of Buenos Aires province to the labor- based Peronists.

''Big gorilla, get out of Government House,'' chanted several hundred Peronists revelers as they marched on the rose-colored presidential palace in downtown Buenos Aires. ''This is the house of (the late President Juan) Peron.''

The overall results indicated wide dissatisfaction among the 19 million electorate with the nation's economic malaise and repudiation of the amnesty granted in June to 270 military officers accused of human rights atrocities during the 1976-83 ''dirty war'' the military waged against leftists.

The Peronists were strongest in the gubernatorial races, winning key Buenos Aires province and 15 other governor's contests for a gain of four seats from the Radical Civic Union Party since 1983.

The six remaining governor's posts were split between local parties and the Radicals, who lost their governorships to the Peronists in Mendoza, Entre Rios, Chubut and Misiones provinces.

With 66 percent of the returns counted, official results showed Peronist gubernatorial candidate Antonio Cafiero had upset Juan Manuel Casella of the governing Radicals 44.6 percent to 36.8 percent to win populous Buenos Aires province.

Cafiero is now considered the top 1989 presidential candidate of the Peronists, who had suffered debilitating defeats to the Radicals in the elections of 1983 and 1985.

Nationwide results showed the Peronists leading with 41.1 percent of the votes to 37.9 percent for the Radicals. The small, rightist Central Democratic Union had 6.3 percent, followed by the leftist Intransigente Party with 2.1 percent and several far-left parties.

At stake in the balloting were half the 254 seats in the House of Deputies, 21 of the 22 provincial governorships and almost 10,000 municipal posts.

The official Telam news agency said about 80 percent of eligible voters cast ballots.

Before the balloting, the Radicals held 130 seats in the House of Deputies. The Peronists have 103 house seats, but control the Senate. The Intransigente Party has six house seats, the Central Democratic Union three and provincial parties 12.

The Radicals held seven governorships, the Peronists 12 and local parties three, including San Juan province, which held its election Aug. 2.

For the first time in 25 years, elections determined all 22 governors, one governor having been elected earlier. Military coups and canceled elections blocked that balloting since 1962.

The Radicals won 51.9 percent of the vote to upset the Peronists in the election in 1983 when Alfonsin came to power, ending more than seven years of military dictatorships.

Several of Alfonsin's legislative projects are in jeopardy now that his party has apparently lost its house margin over the opposition, including plans to change the constitution. Constitutional reform requires a two-thirds majority vote by Congress.

Chief among Alfonsin's plans would be a change to allow incumbent presidents to stand for re-election and to allow a non-Roman Catholic to become president or vice-president. Argentina's official religion is Roman Catholicism.

The constitution was last changed in 1949 to allow the re-election of President Juan Peron, but was nullified by the military regime that ousted Peron in 1955.

After his election, Alfonsin gained international acclaim for putting military leaders on trial. Five former military leaders, including two former presidents, were convicted in connection with human rights abuses and are serving terms of up to life in prison.

At least 9,000 people vanished without a trace during the ''Dirty War.''