MANILA, JULY 13 -- A Peace Corps worker has been kidnapped. The president of the American Chamber of Commerce has left the country after being targeted for assassination by Communist rebels. And today, a series of bombs rocked Manila's financial and tourist districts.

These and other incidents linked to acts of political violence by Communist and right-wing rebels here have prompted rising apprehension within the U.S. diplomatic and business community and are altering the way American citizens in Manila live.

Certain nightspots are now avoided. Work hours are staggered. Routes are varied to throw off possible surveillance. Businessmen are taking summer vacations early -- and some are considering whether to leave the Philippines permanently.

"I've never before heard so many people discussing their options," said one American businessman here who is active in the American Chamber of Commerce. "Some corporate headquarters here are rethinking the value of staying. . . . And I hear of organizations finding it more difficult to recruit people to come here."

Political violence and the fear it provokes are rising despite efforts by the government of President Corazon Aquino to calm the situation and attract foreign investors. Communist assassination teams have murdered nearly 80 people in Manila this year, mostly Philippine military officers, policemen and government officials. Two American airmen were killed in Angeles City. In addition, right-wing military rebels who nearly succeeded in toppling Aquino last December are believed to be working underground against her government.

On the other hand, many Americans here say they do not feel threatened in Manila, a city of about 8 million people. And many American diplomats, recently awarded "danger pay" to be posted here, say Manila is preferable to many other troubled capitals.

But the concern among many increased today when six bombs exploded before dawn in Manila's tourist belt and in the financial district of Makati, which are heavily frequented by Americans and other foreigners. No one was hurt in the simultaneous blasts, which did not appear intended to cause serious injuries.

Police and troops were put on full alert after the bombs, some of them placed in cars and most of them military-issue plastic explosives, went off at several hotels, a department store and a government building.

A television station quoted an unnamed rebel military source as taking responsibility for the attacks in the name of the underground Reform the Armed Forces Movement, which launched last year's coup bid. The source said the bombings were intended as a training mission for recruits in advance of a new coup try.

For Americans, however, a more serious fear is that Communist urban guerrillas will launch another anti-American attack as the August date approaches when U.S. and Philippine negotiators open a new round of talks on the future of two large American military installations, Clark Air Base and Subic Bay Naval Station. So high is the apprehension that U.S. officials are hoping to withhold from the public the date of the talks to avoid giving the radicals time to organize an attack.

The opening of talks last May was marred when Communist rebels, who have called for the pullout of American military bases, assassinated two U.S. airmen outside Clark and then threatened more attacks.

Since then, however, diplomatic and intelligence sources say the rebels have had difficulty finding targets. Protection for diplomats in Manila was increased, and the base towns of Olongapo and Angeles City have become virtual armed camps. As a result, the intelligence analysts believe, the rebels have broadened their list of targets to include American Peace Corps workers and some businessmen.

The 261 Peace Corps volunteers here were pulled out of the country last month after it was learned through Western intelligence channels that more Americans were potential targets. U.S. officials then discovered that one of the volunteers, Timothy Swanson, had already been abducted by rebels on Negros island. He is still being held.

Last week, intelligence sources learned that Mark Blacker, president of the American Chamber of Commerce and a vice president of American Express Bank, was listed as an assassination target. Blacker left early for a scheduled trip to Hong Kong for medical treatment, and it is unclear whether he will return.

One leading U.S. businessman here said other Americans in the business community had also received threats.

Last Friday, the U.S. Embassy warned that it had an unconfirmed report that rebels may try to kill an American on the tourist "strip" known for its go-go clubs.

The area has been declared off-limits for Marine guards from the U.S. Embassy, and all Americans were advised to take precautions. Many Americans have chosen to stay away -- as have Europeans and Australians afraid of being mistaken for Americans.

Analysts said the threat against Americans is a sign that the left's strength is declining. The rebels, the analysts said, are turning to more dramatic urban attacks to keep their movement visible.