A sweeping victory by right-wing forces in Portugal's general election yesterday has changed profoundly the political face of the country just six years after a young officers' coup propelled the left to power.

Conclusive vote results available today gave the ruling Democratic Alliance coalition a comfortable majority for a four-year term and a clear mandate to turn back the clock on the April 1974 revolution.

With almost all the returns of the 85 percent turnout accounted for, provisional results showed that the ruling coalition increased its majority in the 250-seat assembly, winning 136 seats against 128 won in last December's vote. It increased its percentage of the total vote from 42 to 47 percent. The Socialists, despite a vigorous campaign, dropped one seat to 73 and the Communists slumped from 47 seats in December to 40. The extreme left retained control of one seat.

As fireworks exploded throughout the night, jubilant supporters honked their way through Lisbon's streets in endless noisy, flag-waving motorcades. It was clear that the landslide victory was a personal triumph for the Democratic Alliance's leader, Premier Francesco Sa Carneiro, and a potentially deadly blow to President Gen. Antonio Ramalho Eanes, who is backed by the Socialists.

Eanes, a bitter political opponent of Sa Carneiro, remains the last obstacle in the way of the premier's program to overhaul radically the revolutionary Portuguese constitution. Framed in the heady days of 1975, the constitution pledges that Portugal will carry out "the transition to socialism by creating the conditions for the democratic exercise of power by the working classes."

With the support of the left, Eanes is seeking reelection to a second term in a presidential vote due in December. A confident Sa Carneiro predicted at a dawn press conference that the president would be defeated by the ruling coalition's candidate, conservative and largely unknown Gen. Antonio Soares Carneiro (no relation to the premier).

"These elections are the first round of the presidential elections and they mark a defeat for President Eanes, who identified with the Socialists," Sa Carneiro said.

The unexpected winner of a midterm election last December with a six-seat majority, Sa Carneiro, 46, ran into the opposition of both Eanes and the military council of the revolution when he attempted to put his pragmatic rightist policies into action.

At the press conference Sa Carneiro, a successful lawyer who entered politics in 1968 when he was a rare opposition deputy who won toleration in the parliament of the former rightist dictatorship, reiterated his threat that he would resign should Eanes be reelected in the presidential vote.

The renewed swing to the right in yesterday's vote, which according to Democratic Alliance officials surpassed their expectations, appeared to underscore the profound disenchantment of the 7 million Portuguese voters with the leftist and revolutionary experiments of the recent past. Since April 1974, Portugal has consolidated its unenviable position as Western Europe's poorest nation as average real wages slumped by approximately 20 percent.

Largely rural, strongly influenced by the Roman Catholic Church and with an illiteracy rate of approximately 30 percent, Portugal has voted twice in less than a year for a stable conservative majority in parliament. There have been 12 governments in six years, ranging from communist-back military juntas to shaky, socialist-led coalitions.

Eanes, credited with ridding the revolution of its worst excesses after he came to power as a virtually unopposed compromise candidate in 1976, appears to be the principal loser of the election. But Socialist Party leader Mario Soares, a former premier and a leading post-revolutionary politician, looks certain to face increasing unrest within his own rank and file, where he is likely to be seen as a consistent loser.Difficulties also would appear to lie ahead for the hard-line leader of the pro-Soviet Portuguese Communist Party, the veteran Alvaro Cunhal.

In its 10 months in power, the government has been markedly anticommunist, and rightist leaders suggested early today that the poor showing of the Communists was the result of voters' associating the party's policies with those of the Soviet Union. In what was the first electoral test of a European pro-Soviet party since the invasion of Afghanistan, and with Polish worker unrest fresh in the minds of the voters, the Portuguese Communists dropped 2.5 percentage points to 17.5 percent of the vote. Last December, at the expense of the Socialists, they had increased their 1976 share of the vote from 14.6 to 19 percent.

It was characteristic of the new mood that a former minister of the rightist dictatorship gained a seat, something nobody would have predicted as little as a year ago.

The return to national politics of such associates of the previous reign funneled fears on the left that the revolution would not just be stopped in its tracks, but that the clock could be wound right back to the dictatorship.

There were reports today of police being called in to protect leftist militants from rightists who attempted to storm their headquarters in the industrial town of Oporto. Socialist leader Soares said early today, "We know how to lose; I hope the Democratic Alliance knows how to win." Communist Alvaro Cunhal warned of an upsurge in neo-fascism, which he said his party would resist by all means possible.

Sa Carneiro rebutted such charges, indicating that he plans to embark on a policy of ever closer ties with the West in general and the United States in particular. The trend since last December, in which Portugal has been one of Washington's most reliable NATO allies, seems certain to continue.

Last January, Portugal froze its relations with Moscow after the invasion of Afghanistan, and later boycotted the Olympics and became the first NATO ally to impose economic sanctions against Iran. This year there have been preliminary bilateral discussions aimed at ceding a Navy and aircraft-carrier base in mainland Portugal to the United States, and talks toward granting facilities at an air base south of Lisbon and increasing the U.S. presence at the Lajes base in the Portuguese Azores.