In his clearest statement to date on the Philippine presidential campaign, the influential leader of the Roman Catholic Church here, Cardinal Jaime Sin, has warned of a "sinister plot" to cheat in next month's election, undermining its validity.

In a pastoral letter released today and scheduled to be read in churches across the country Sunday, Sin called on Filipinos to forswear participation in any "election bribery, cheating or violence." His comments are expected to have a major impact on Filipinos, more than 80 percent of whom are Roman Catholics.

While Sin did not identify those he said already were engaged in "evil tactics" in the current election campaign, the letter made it clear that the accusations were directed against the ruling party of President Ferdinand Marcos.

The letter appeared to buttress charges by supporters of opposition candidate Corazon Aquino that Marcos' New Society Movement is preparing to cheat massively in the election, which is set for Feb. 7.

Aquino supporters say that while the church has not taken a public stand on the election, it discreetly supports her candidacy.

In recent days, Aquino supporters have accused Marcos lieutenants of trying to intimidate voters, engaging in dirty tricks against the opposition, bribing teachers who serve as election inspectors, padding voters' lists in certain areas, gerrymandering, denying air time on television and radio to the Aquino campaign, using government funds for election purposes and pressuring government employes to vote for Marcos.

In addition, Marcos has been criticized for using the powers of his office for questionable electioneering practices such as disbursing funds for various projects on his campaign stops, distributing land on military reservations to squatters, and decreeing reductions in taxes and electricity rates.

As part of what the opposition sees as measures to weaken resistance to election fraud, the government and ruling party lately have been bitterly attacking the National Citizens' Movement for Free Elections, a poll-watching group known as Namfrel, and demanding the resignation of its chairman, Jose Concepcion Jr.

The government has accused the organization of receiving funds from the United States, notably the Central Intelligence Agency, and of being biased in favor of the opposition.

A major target of the campaign has been Namfrel's plan to conduct an "operation quick count" using computers and a nationwide network of poll watchers to tabulate returns as they come in from the country's 90,000 precincts. A similar operation was credited with limiting cheating by Marcos' party in legislative elections in May 1984, when the opposition made a surprisingly strong showing.

In his pastoral letter, Sin said next month's election was of "decisive importance" for the Philippines. "But we already see many signs that show a very sinister plot by some people and groups to frustrate the honest and orderly expression of the people's genuine will," he said.

"Already money has flowed freely into the hands of teachers, barangay ward officials and the common people to induce them to support a particular candidate in a manner unworthy of free persons. Already we have seen, heard and read lies and black propaganda used by some quarters against opponents who are on the other hand deprived of adequate access to media, and are thus unjustly left defenseless. Already we hear of undue pressure exerted on hapless government employes to make them work against the law for certain candidates."

Deploring "such evil tactics," Sin called on Filipinos to "desist from evil and to resist evil." He urged them to "make of this election not one more national scandal and offense to God but a new beginning; . . . otherwise we court national disaster."

Addressing the "poor and oppressed," the letter said: "Those who have victimized you in the past will want to continue victimizing you. They will continue to use money, persuasion and intimidation for their selfish ends. Please resist them and do not cooperate in your own oppression." Instead, the letter urged Filipinos to join vigils at polling places and expressed the church's "strong support for Namfrel in a special way."

Sin, the archbishop of Manila, often has incurred the wrath of the Marcos administration for his outspoken criticism of its alleged corruption and abuse of power. He once compared the Marcos government to "a plague of locusts."

Among the election irregularities recently discovered by Namfrel was an apparently padded voters' registration list in Makati, a "city" within metropolitan Manila. Namfrel's Concepcion estimated this month that there may be 73,000 "flying voters" in Makati who have registered illegally a number of times. He said that out of 933 voters' affidavits examined after a 1984 registration, 616 were found to have been made by only 60 persons.

Concepcion also cited "the incredible phenomenon of large numbers of people living in the same house" in Makati. For example, he said, records showed 206 persons living in one Makati house, with 147 occupants next door.