Tens of thousands of Chileans gathered to hear Gen. Augusto Pinochet proclaim victory tonight and Santiago's upper-class neighborhoods erupted in celebration after the government announced that it had won today's constitutional plebiscite by a margin that is expected to exceed 65 percent.

The government's apparently overwhelming victory was questioned, however, by hundreds of denunciations of irregularites and fraud to opposition radio stations throughout the day, indicating a consistent pattern that might well have invalidated an election in the United States or Western Europe.

Pinochet, Chile's military president, attributed the victory, which will keep him in power for at least nine and possibly 17 more years, to the good sense of Chileans. He said they had voted against international communism and "demonstrated their devotion and their desire that the military government continue."

With about 40 percent of the vote counted as of 10 p.m., the official results showed a 69 percent "yes" vote in favor of the proposed new constitution, which provides for a still sketchy form of "protected democracy" at the end of Pinochet's rule. About 28 percent were "no" votes, and about 3 percent invalidated ballots.

The denunciation of fraud included events at a high school in La Reina, a Santiago suburb, where people waiting to vote in the afternoon were reportedly told to leave the premises and return half an hour later. When the voters returned, the ballot boxes had been opened and apparently tampered with.

At a school in one of Santiago's poorer sections, where the "no" vote might have predominated, voters were given pens to mark their ballots. When the votes were counted later in the day, those marked in ink were reportedly declared void.

At the Christian Martinez school in San Bernardo, the chairman of one voting table simply stuffed all of the unused ballots into the ballot box, according to eyewitnesses. Blank votes were considered by the government to be the same as "yes" votes.

The draft charter was written by supporters of President Pinchet, who seized power seven years ago and now seeks to remain in power until 1989.

Opponents of the draft constitution led by Eduardo Frei, the Christian Democrat who was president from 1964 until 1973, called the plebiscite A joke," pointing out that no dictatorship had ever lost such a vote. Voting was mandatory today, with citizens risking fines if they did not.

In the one major speech Frei was allowed to give during the month leading up to today's vote, he predicted that approval of the constitution and a prolongation of Pinchet's rule could lead to extreme political polarization, violence and possibly civil war.

Except for a January 1978 referendum carried out by the current military government, Chileans had not voted since 1973, the last time elections were carried out under a democratic government.

To improve its chances, the government mounted a massive propaganda campaign playing on widespread fears of a return to the economic and political disorder that existed before the 1973 coup, gave opponents little chance to present their case and designed a ballot-counting system that could allow fraud.

In addition, Pinchet threatened to return to his barracks, leaving the country without viable government, should the proposal fail.

The plebiscite, which Pinchet unexpectedly announced only a month ago, was set for this seventh anniversary of the coup that toppled Salvador Allende, Chile's last elected president and then the only Marxist chief executive to come to power as a result of free electins.

Many Chileans genuinely fear a return to the near-civil war of Allende's last month in office.

Although today's voting, carried out at 33,000 booths, was peaceful, there have been increasingly large and vocal antigovernment demonstrations in Santiago each night this week and it was understood that the government has quietly ordered Army reserve units to be ready for duty Friday, when the final results are to be announced.

Nonetheless, the Pinchet regime and its civilian partisans remained determined to ensure the general's continued rule through the end of the decade or possibly until 1997 if the military junta nine years from now should decide that the country was not ready for elections.

The constitution, if approved, woudl not be activated until 1989. Under terms of the "protected democracy" embodied in it, the current capitalist economic system could not be changed, the once-powerful Communist Party, as well as all Marxist rhetoric, would be outlawed and political parties such as The Christian Democrats would have a reduced but as yet unspecified role in future elections.

One article of the draft constitution would give Pinochet the right to ammend it between now and 1989 if he chooses, with approval of the rubber stamp military Junta, which would serve as the legislative body to Pinochet's executive during the contemplated nine-year transition period.

The Roman Catholic Church hierarchy in Chile and the opposition Christian Democrat Party, which is officially proscribed but which the government allows to function, both publicly expressed fears that the result of today's vote could easily be manipulated by the government.

They pointed out that there are no current electoral registers, meaning no list of those eligible to vote.