Opposition presidential candidate Corazon Aquino said today that a special envoy President Reagan is sending to the Philippines would be welcome, but she suggested that U.S. policy could be helping push the country toward catastrophe.

In a written statement, Aquino urged Reagan not to "conspire" with President Ferdinand Marcos "to cheat the Filipino people of their liberation."

Aquino, who has claimed victory over Marcos in Friday's election, said she could never be reconciled with him, as officials in Washington have suggested be tried. "Too many people will be dead the moment the world's head is turned," she said.

Her words matched equally strong sentiments among many of her followers. They charged that the United States is betraying a commitment to foster democracy in the Philippines and is tilting toward accepting a fraudulent Marcos victory as the counting of votes from the election continues.

The statement came as the opposition appeared to be losing hope that Aquino can beat Marcos within the system. Many members of her camp have complained that the U.S. stand is increasing Marcos' freedom to manipulate the vote. Marcos has denied doing so.

The National Assembly, dominated by Marcos' New Society Movement party, today continued the tedious process of opening boxes with voting returns. The legislature is widely expected to proclaim Marcos the winner, perhaps later this week.

Aquino said Reagan's envoy, Ambassador Philip C. Habib, would "no doubt" be welcomed. "I must confess to some alarm, however, that his last task for the president was trying to negotiate an end to Lebanon's civil war," she said. "I hope neither Mr. Reagan nor Mr. Marcos is expecting to see our beloved country go the same way."

A spokesman for Marcos, meanwhile, said the president would welcome Habib's trip as a chance to increase U.S. understanding of the situation here.

U.S. Ambassador Stephen Bosworth called at Aquino campaign headquarters yesterday and again today, but there was no word on the substance of his talks.

Aquino said today that a fair count would show she took between 80 and 90 percent of Friday's vote. But she said her lead was whittled down by such things as fraud, intimidation and deliberate delays of the voting process.

A tally by the government's Commission on Elections showed Marcos in the lead, 6.4 million to 5.9 million. A separate incomplete count by the citizens' group known as Namfrel showed her leading Marcos, 7.2 million to 6.5 million, tonight.

Anger with the United States in the Aquino camp was sparked by statements earlier this week by Reagan and other senior government officials about the election, which was observed by an official U.S. team.

In a press conference yesterday, Reagan said the U.S. will withhold judgment on the fairness of the election until the outcome is final. He expressed concern over "reports of fraud" but said that both sides might have been doing it. He also underlined the importance that the United States places on its air and naval bases here.

Although his remarks were portrayed in Washington as stepping back from his statements Monday, his failure to condemn Marcos for electoral fraud and his suggestion that the opposition also was guilty of it kept emotions here high.

Aquino aides also expressed indignation over advice from an unnamed administration official on Monday that Filipinos should not stage street demonstrations if they don't like the outcome of an election. Aquino has said she will call for street protests if she is declared the loser.

"I suggest to him [Reagan]. . . that before making further comments on the election itself, he make additional inquiries from his own embassy, the observers and the media," Aquino said.

She rejected suggestions from U.S. officials that the two sides could be reconciled. "It would also be a delusion of power to believe that an opposition whose leaders and followers have been and are being killed can suddenly settle down to a western-style opposition role in a healthy two-party system," she said.

"I would wonder at the motives of a friend of democracy who chose to conspire with Mr. Marcos to cheat the Filipino people of their liberation," she added.

In Aquino campaign headquarters today, one worker was wearing a button that he said he had earlier discarded. It read: "Dismantle U.S.-Marcos Dictatorship." It was indicative of the mood of anger and bitterness that has overtaken many Aquino supporters due to Reagan's statements.

Phone calls of protest have been received in large numbers at the U.S. Embassy, according to a spokesman there.

Many in Aquino's camp had come to see Washington as a partner, usually silent, but given to speaking up at strategic moments in their fight to end Marcos' 20-year grip on power.

Although Filipinos are loath to admit it, political and moral support from the United States, which ruled the country from 1898 until 1946, is key to the maintenance of legitimacy in power here.

"America was the one that taught us all about democracy," said an Aquino campaign worker. "People still look up to it." Philippine democracy is often rated by how it compares to that practiced in the United States.

For years, Marcos clearly had the U.S. stamp of approval. But last year, a distancing began. A stream of visitors from Washington and statements from the State Department led many Filipinos to feel that the Americans wanted Marcos out and would help get him there.

In conversations today, Filipinos questioned whether the United States understands the extreme polarization that has emerged here in a campaign that has left more than 90 persons dead. Some officials at the U.S. Embassy here have raised the same questions.

Aquino appears to believe firmly that Marcos was behind the 1983 murder of her husband, opposition leader Benigno Aquino. She has suggested just as strongly that Marcos is now stealing an election victory from her.

Aquino and many of her people appear to feel that it will take more than an election to get Marcos out. But she is said to have placed little faith in help from the Americans. She is said to believe that had Washington tried harder, it could have prevented the murder of her husband.

Thus, campaign sources say she was not surprised over the U.S. statements. It was worse news for some of the advisers around her, who had hoped for pressure from the United States and the offer of a place of exile for Marcos to help form his eventual decision to leave.

Marcos called the election partly because of U.S. pressure for reform. Campaign sources said that Aquino and her advisers feel that, although ill-prepared for an election, they risked everything only to have the Americans pull the rug from under them now.

Talk is already surfacing in Aquino circles that Reagan has made a strategic mistake. "If his purpose is to help President Marcos," opposition legislator Ramon Mitra said yesterday, "he will have to send the American Army here to enable Marcos to govern. Because he can't. He has no mandate."