ASUNCION, PARAGUAY, FEB. 14 -- Gen. Alfredo Stroessner, already the Western Hemisphere's longest lasting ruler, today extended his presidential reign for an eighth term in an election branded fraudulent by opposition groups.

Clusters of demonstrators in various cities braved antiriot police forces to protest the lack of democratic freedoms. Several opposition leaders, among them Domingo Laino, were detained briefly for trying to promote an election boycott.

Some foreign observers reported seeing irregularities, including people voting more than once, a lack of secrecy in polling places and an absence of ballots for some parties at some tables. A Paraguayan newspaper columnist, who went to vote 10 minutes before his local polling place closed, said he was told his name had already been checked off as having voted.

Using the measures that Stroessner's critics say are common here, state security agents clubbed a group that included leading dissidents from the ruling Colorado Party, foreign journalists and a visiting member of the U.S. Democratic Party's foreign affairs institute as they tried to enter the town of Ypacarai, a focus of dissident activity.

Although refusal to vote is subject to a fine, independent spot checks in numerous districts late today found an abstention rate of 10 to 20 percent. Opposition parties had urged voters either to abstain or to cast blank ballots.

With most of the ballots counted, the electoral center at the Colorado Party headquarters gave Stroessner 89 percent of the vote for another five-year term. Dropping his own ballot in a box this morning, the 75-year-old Army general, who has governed since 1954, said, "This is democracy," and pledged to devote "my energy to the national good."

On a continent that in this decade has seen most other countries restore or maintain democratic rule, Paraguay keeps to an autocratic order. But with the wane of the Stroessner era widely considered to be under way, there are mounting signs of maneuvering both inside and outside the government for influence over the succession.

Since seizing power 34 years ago, Stroessner has ruled by repressing his critics and bestowing favors on his fellow generals and civilian bosses in the Colorado Party. Backers extol the political stability and economic growth that the Stroessner years have brought, particularly in contrast with the warring and chaos that ravaged Paraguay earlier this century.

But a recent economic slowdown, following completion in the early 1980s of the mammoth Itaipu dam on the border with Brazil, has sapped the government's strength. Street marches protesting low salaries and political oppression erupted in 1986 for the first time in years.

The country's Roman Catholic bishops have started pressing for a transition to democracy as have business leaders demanding economic modernization. The United States has become more critical of Paraguay's human rights violations and performance in controlling drug traffic.

Even the Colorado Party, long a monolith of power with 1.4 million members in a nation of 3.7 million people, has split into bitter factions over the issue of one-man rule. Militant supporters of Stroessner ousted so-called traditionalists from influence last year.

Despite efforts to inject spontaneity into what virtually has become a ritualistic process, Stroessner's reelection campaign was dogged by a sense of exhaustion. At the final party rally here Thursday night, pro-Stroessner forces failed to pack Independence Plaza. Organizers claimed 150,000 attended but diplomats and other independent observers put the number at under 50,000.

Stroessner addressed only three major campaign rallies in the last six weeks. At the last one, he read a perfunctory message on the gains of the past three decades -- more schools, hospitals and power transmission lines.

Two other candidates representing the tiny parties that participate in government with the Colorados provided token opposition to Stroessner in today's balloting. A man-on-the-street survey in a local newspaper found no one able to give the name of either. The two, Carlos Ferreira Ibarra of the Liberal Party and Luis Maria Vega of the Liberal Radical Party -- also ran for Congress.

The official election result contrasted sharply with a nationwide political opinion survey by the Catholic University in late January. The poll showed 43 percent intending to vote for the Colorado Party, 31 percent planning to abstain and 11 percent saying they would cast blank ballots. Another survey taken by the university three years ago produced similar breakdowns in political leanings.

Asked in the recent poll if a "climate of unrest" existed in Paraguay, 53 percent said "yes," and a majority blamed either the government or the Colorado Party leadership for it. Asked to name the person most capable of leading Paraguay, 48 percent declined to respond. Among those who answered, Stroessner ranked first but Laino, 52, an attorney and leader of the officially unrecognized Authentic Liberal Radical Party, came in second.

"We could say that the citizenry has assumed more the role of a spectator than a protagonist," concluded the study. "It is as if the public attends a spectacle without the power to affect the outcome."

Attempts by Paraguay's small opposition groups to build their bases of support have been frustrated by police repression of public rallies and frequent temporary detentions of leading dissidents. Political and personality differences among the opposition also have gotten in the way of a unified strategy for challenging Stroessner.

Efforts by the Catholic Church to promote unity have reached an impasse. A church-sponsored "national dialogue" drew together political, labor and social groups -- except the Colorado Party, which refused to participate. Its report in December was highly critical of Stroessner's regime. "The church reached a point where it didn't know what more it could do with the dialogue process," said Ilde Silvero, editor of the episcopate's daily paper, Sendero.

Given the opposition's weak condition, many here see the main hope for democratic transition resting with dissidents inside the Colorado Party