Radical Party presidential candidate Raul Alfonsin claimed victory here early this morning in national elections for a new civilian government that indicated a historic shift in Argentina's political balance after nearly eight years of military rule.

With nearly 50 percent of electoral stations officially counted, Alfonsin, a moderate center-left leader, had taken a strong lead over his principal rival, Italo Luder of the Peronist or Justicialist party. Alfonsin had won 55 percent of tabulated votes, against 37 percent for Luder, and was leading in every major electoral region in the country.

If sustained, the margin of the 56-year-old lawyer and former congressman would mean a stunning defeat for the populist Peronists, who dominated Argentine electoral politics for more than 30 years. It would also break a lengthy political cycle in which Peronist and military leaders have alternated in power amid continual national instability.

Peronist leaders refused to concede defeat, however, and instead bitterly attributed Alfonsin's early lead to manipulations by the military government-run tabulation center. "A gigantic informational fraud is being carried out in which official information which indicates a Peronist victory is being held back," said Lorenzo Miguel, the party's top official. He added that the alleged maneuver was meant "to lead the public to a civic confrontation and alter the democratic process."

Radical party leaders said the preliminary results far surpassed their most optimistic expectations. They predicted that Alfonsin would win a majority of the popular vote, virtually assuring his election to a six-year term by the electoral college on Nov. 30. "We have been waiting a long time for this triumph," said Enrique Vanoli, one of Alfonsin's campaign directors. "We have a new leader in Argentina."

Alfonsin's backers have described him as a potential successor as a charismatic national leader to Juan Domingo Peron, a three-time president whose death in 1974 ignited a struggle for leadership in his working class-based party that has never been fully resolved.

Jubilant Alfonsin supporters took to the streets of the capital by the tens of thousands last night to sing, chant and blow horns. Several minor violent incidents involving Peronist party militants were reported by early this morning.

Alfonsin told thousands of cheering supporters from the balcony of his campaign headquarters: "We have won, but we defeated no one. . . this is a triumph of all Argentina," Reuter reported.

Long lines of peaceful voters formed early on a clear spring day in the capital and other major cities, and only minor irregularities were reported by last evening. Nearly 18 million persons in Argentina's population of 28 million were eligible to vote for president, vice-president and a 300-member, two-chamber Congress as well as state governors, legislators and local officials.

The election represented Argentina's third return to democratic government in the past 20 years and only the fifth democratic election of a president since 1950. All four previous governments were ended by military coups. The armed forces last took power in March 1976, overturning the government of Isabel Martinez de Peron. They initiated a harsh right-wing rule marked by thousands of abductions and killings by security forces and ambitious efforts to restructure the economy and the political system.

A severe recession beginning in 1981 and the military's disastrous invasion of the Falkland Islands last year forced a hastened return to democracy. The ruling junta lifted a nine-year-old state of siege on Saturday and plans to transfer power to the new government as early as December and no later than Jan. 30. The transition will take place at least three years earlier than military leaders once intended.

The new government will inherit a shattered industrial base, the world's highest inflation and a $40 billion foreign debt that has brought Argentina close to financial default. The top presidential candidates also said they would confront severe difficulties in dealing with the military's legacy of human rights violations--including thousands of unresolved cases of disappearance--and a continuing diplomatic impasse with Britain over the Falkland Islands.

The sense of crisis made the consolidation of democracy a predominant theme of the two-month electoral campaign and raised public enthusiasm for the elections to an unprecedented level. A record percentage of Argentines became affiliated with political parties during the last year, and appearances by Luder and Alfonsin commonly drew crowds numbering in the tens of thousands.

As in past Argentine elections, the competition between the major parties bore little relation to traditional divisions of ideology or constituency. Alfonsin, who is sometimes likened to a European social democrat, is considered more liberal than Luder, a 66-year-old constitutional lawyer and former Senate president. However, Alfonsin was supported by Argentina's traditional civilian conservative parties while Luder was endorsed by the Popular Socialist and Communist parties.

Though the Peronist party is based largely in Argentina's urban working class and poor, the business community was divided by the election and many large corporations favored the Peronists. The Radicals and Peronists issued nearly identical economic platforms emphasizing aggressive state intervention to raise salaries and employment and to stimulate exports and industry.

In a country plagued by coups and internal violence for more than half a century, the Radicals and Peronists divided on more fundamental issues of style and approach to government. And in an often bitter campaign, debate on issues was all but nonexistent as each party sought to establish the other as antidemocratic or as linked to the discredited military and "foreign interests."

Alfonsin gained the image of a strong leader who offered a fresh alternative to frustrated, disillusioned voters.