A three-day-old boycott called by opposition leader Corazon Aquino has begun hurting businesses associated with President Ferdinand Marcos, reinforcing opposition hopes that the campaign will help topple his government following this month's disputed election.

Businessmen here said today that firms targeted by the campaign had suffered an immediate drop in sales after the boycott call and that many prominent businessmen had resigned from a presidential advisory council in protest against the government.

Aquino pledged today to continue and expand the campaign, launched Sunday, until Marcos resigns. While businessmen in Manila say it is too early for precise figures on the boycott's effects, several said in interviews that popular enthusiasm for the campaign and business leaders' dissatisfaction with the Marcos administration are forcing the business community to take sides in the postelection political battle.

The Philippine peso held today at 22.04 to the U.S. dollar following its fall from 19.98 yesterday, its biggest single-day drop in years.

Members of the opposition said they want the boycott to demonstrate Marcos' isolation from the majority of Filipinos, and thereby encourage the military, civil service and the international community to abandon support for the 68-year-old president.

Aquino has asked her supporters to withdraw their money from banks and boycott pro-Marcos newspapers and other businesses run by the state or friends of Marcos.

Speaking today at a rally in Angeles, about 50 miles north of here, Aquino and other opposition leaders claimed the boycott had produced heavy withdrawals from some of the seven targeted banks and had hurt sales of the San Miguel Corp., run by a close friend of Marcos. An editor at Bulletin Today, one of the targeted Manila newspapers, said the newspaper had lost a significant amount of advertising.

"The very preliminary indications are of heavy withdrawals from Coconut Planters' Bank, Commercial Bank of Manila and Security Bank -- and a branch of another bank even had to close at the university campus," said Jaime Ongpin, an aide to Aquino.

Officials of several of the targeted firms could not be reached for comment.

Aquino's supporters and independent analysts suggested that the current campaign is more effective than similar efforts in the past.

Omar Cruz, an economist at the Center for Research and Communications, an independent think tank, cited sources at targeted newspapers, for example, as saying they had suffered a substantial drop in circulation.

Eduardo Sanchez, a bank executive sympathetic to Aquino, said, "People feel they've made a very large investment in trying to hold free elections . . . . This is why Mrs. Aquino's call has had such a tremendous response."

Arlene Buan, a lawyer attending Aquino's rally, said he withdrew his savings from the Philippine National Bank this morning, and cited friends working for the bank who had told him its Angeles branch had lost 3.5 million pesos -- about $175,000 -- in deposits on Monday alone.

Many Filipinos have argued, with a smile, that the boycott against San Miguel will be the hardest to enforce because the firm brews the country's best selling beer. San Miguel, the nation's largest manufacturing company, is the Philippines' only bottler of Coca-Cola, also subject to the boycott.

While they predicted difficulties for the companies targeted by the boycott, none of the businessmen suggested the firms' survival might be threatened. Stock prices for the San Miguel Corp. stabilized today after having fallen sharply Monday with the news of the boycott.

Economist Cruz and several businessmen and economists argued that the continuing alienation of the business community is more important than the direct effect of the boycott. Cruz said, however, that "it will take quite a while before this snowballs into a force which could remove the president."

A number of prominent businessmen underlined their despair with the government by resigning earlier this week from the advisory President's Productivity Council. Several denied a government claim that they had quit as a courtesy to Marcos, and insisted their move had been a protest against his government.

One who resigned, Philippine Chamber of Commerce and Industry Director Felix Maramba, said relations between the business community and the government have eroded steadily over the past two to three years. He said businessmen had been stunned by the amount of money spent by the government during the election campaign and the steep increases in interest rates ordered this week by the Central Bank.

"This crisis of confidence is the same as what happened afer the assassination of former opposition leader Benigno S. Aquino Jr. " Maramba said. "Never in our wildest dreams did we imagine this." Maramba joined other economists in predicting a net shrinkage of the national economy this year.

"We have reached the point where very few can afford to stay neutral in politics," Sanchez said.