PARAGUAYAN WINS HIS EIGHTH TERM

The Government announced today that Gen. Alfredo Stroessner had been overwhelmingly elected to an eighth presidential term, and the opposition charged the voting was marked by widespread fraud.

The outcome of today's election was never in doubt, but early results seemed to reflect the Government's desire to demonstrate the failure of an opposition campaign calling on voters to abstain or to annul their ballots.

While opposition spokesmen asserted that abstentionism had exceeded 50 percent in many urban areas, the Government was announcing voter turnouts of between 90 and 98 percent, even in areas where the opposition is known to have support. Official Vote Count

In a speech broadcast on radio and television, the Interior Minister, Sabino Montanaro, said that General Stroessner and his party's slate of candidates won 982,316 votes, or 89 percent of the ballots counted. Senator Luis Maria Vega of the Radical Liberal Party had 78,141 votes, or 7 percent, and Carlos Ferreira Ybarra of the Liberal Party had 32,403 votes, or 3 percent.

Foreign diplomats said it would be impossible to obtain accurate election results because the entire process was tightly controlled by both the Government and the official Colorado Party.

But the diplomats said that, more than during General Stroessner's previous re-elections, the opposition had succeeded in drawing both local and international attention to the authoritarian nature of the regime that seized power here 34 years ago.

The Government has in turn helped the opposition by ordering heavily armed police to break up protests organized by the Committee for Free Elections during the brief election campaign that began after the New Year.

The committee, which includes six small political parties as well as some labor and student groups, had planned to hold a series of anti-Government demonstrations outside churches today, although several were forestalled by police action. Opposition Leader Seized

Domingo Laino, a prominent opposition leader who returned from exile last April, was arrested along with three others before dawn today as they drove to the town of Caaguazu to lead one such protest. They were eventually released in Asuncion 12 hours later.

Two Colorado Party dissidents, Carlos Romero Arce and Waldiro Ramon Lovera, were also stopped by the police as they drove this morning to the nearby town of Ypacarai. Witnesses said both were beaten by police agents and ordered to return to the capital.

In Asuncion, about 100 opposition protesters gathered on the steps of the cathedral after morning mass and held a brief political meeting. The police did not intervene.

Mr. Laino told reporters that, despite what he called today's ''electoral masquerade,'' the opposition would keep up its offensive against the regime during the three months leading up to a scheduled visit here in May by Pope John Paul II. ''After that, I expect the regime to clamp down,'' he added.

Although Paraguay's main opposition newspaper and radio station have both been closed by the Government, the activities and statements of the opposition were broadcast throughout the day by the Roman Catholic Church's radio Caritas, the only independent news outlet that still operates here.

Yet, away from the isolated locations where the opposition was able to make itself felt today, the absence of lines of voters, party flags or even partisan noise made it virtually impossible to know that an election was taking place.

With the temperature in the 90's, the streets of Asuncion were silent and empty. One woman and her aging mother said they had voted for General Stoessner because he guaranteed ''peace and tranquillity,'' but many younger voters said they had abstained, explaining with a smile that they had lost their electoral permits. Some Merit in Voting

One accountant, in contrast, said he had voted, albeit annulling his ballot, because he needed to be able to show the authorities that he had voted when trying to obtain a series of documents and licenses from the Government.

Enthusiasm was not lacking, however, when General Stroessner voted at a school early this morning, with dozens of dark-suited senior officials on hand to applaud him as he arrived in a bullet-proof limousine accompanied by three truckloads of troops.

The ceremony was in fact handled like a military operation. At least 200 soldiers blocked traffic from streets near the school from before dawn, while only journalists with special credentials were allowed to witness the moment that the dictator voted.

''I am going to dedicate all my energy to the nation's well being and all my will, commitment and intelligence to fulfilling this responsibility,'' he told reporters at the scene. ''That order, freedom, democracy and progress reign.''