A terror campaign launched by a new group of political activists is prompting President Ferdinand Marcos to tighten martial law and crack down on opponents.

The violence is the work of a fledgling urban guerrilla movement that symbolizes a spread of middle-class opposition to Marcos after eight years of his marital law rule.

The latest in a series of bombings left an American tourist dead Friday and 32 other persons injured. The bombings, the bloodiest in Manila in recent memory, appeared to signal an escalation of the gerrillas' drive to force Marcos to step down.

According to an urban guerrilla source, more than 500 persons make up the core of the movement, which is supporting by some Filipino dissidents living in the United States. The figure is difficult to verify. What is certain is that the recent arrest and trial of members of the movement's Light a Fire group has not stopped the guerrillas' activities.

It is also apparent that many of the new urban guerrillas have affluent, middle-class backgrounds. Many are successful professionals, and a few have Harvard educations.

A knowledgeable source said there are about 10 groups of urban guerrillas in Manila, with no central command. Small groups coalesce to carry out bombings. Most of the financing and explosives are locally obtained, the source said.

Military intelligence believes that some of the financing has come from the Washington-based Movement for a Free Philippines, a group of exiles dedicated to toppling Marcos.

Strongly anticommunist, the urban guerrillas have shied away from offers of help from the communist New Poeples' Army. But individual guerrillas are believed to have learned to handle explosives from New Peoples' Army members.

The urban guerrillas align themselves to the so-called Third Force, a loose group of social democrates who keep their distance from the extreme left and right.

The urban guerrillas have professed no love for politicians in power before martial law. Their immediate objective is the end of martial law and free elections.

The type of government they want is not clear, but they reject the present elitist system that has dominated political life in the Philippines.

The urban guerrillas gave a series of warnings to Marcos last year. A floating casino, some first-class hotels and department stores -- all belonging to associates of Marcos and his wife -- were gutted.

Parcel bombs were sent to several Cabinet ministers, but they caused no harm. Thus when newspaper executive Eduardo Olaguer and 10 accomplices were arrested last December with a cashe of explosives, his Light a Fire movement was dismissed by some observers as a group of misguided fumbling amateurs.

Now on trial before a military tribunal, Olaguer, 44, admitted taking up arms against the government but pleaded not guilty, claiming that the Marcos administration was illegitimate.

The group's tactics and errors puzzled many. But Olaguer said during a break in his trail that the fires and bombs were meant to be warnings.

Warnings were given again on Aug. 22 and 25 when 11 bombs went off in toilets in government buildings, banks and movies, causing minor dammage. The warnings may have ended when a Los Angeles businessman, Victor Burns Lovely, 35, lost his right arm when a powerful bomb he was tinkering with exploded in his hotel room on Sept. 6. He is still in critical condition.

Two other Americans have been implicated in the guerrilla movement. Steve Psinakis, a relative of the Lopez family, an arch Marcos foe, is being tried in absentia with the Olaguer group. Another, Ben Lim of Seattle, who was caught trying to bring cartons of explosives into the country, died in prision recently from a lingering heart ailment.

A group calling itself the April 6 Liberation Movement took responsibility for the explosions since August. The movement derived its name from a 1978 demonstration organized by opponents of Marcos.

The orgins of the urban guerrilla movement in the Philippines can be traced to 1975 when Marcos began manipulating the constitution to give himself absolute power.

In 1978, opponents accused Marcos of rigging elections in which his new Society Movement party swamped the National Assembly. In early 1979, the first parcel bombs were sent and the first began.

The Philippines' leading churchman, Cardinal Jaime Sin, warned last year of an inevitable civil war if martial law was not lifted soon.

His warnings were repeated by Marcos' chief political opponent, Benigno Aquino, in a speech in New York on Aug. 4. He predicted widespread urban guerrilla warfare if Marcos did not dismantle martial law.

As the bombs began exploding soon after his speech, the government accused Aquino -- who had been in prison throughout the eight years of martial law -- of leading the urban guerrillas. Aquino had been released from confinement in May to undergo heart surgery in the United States.

Predictably, the violence has forced Marcos to tighten martial law. He ordered a crackdown on subversives and stepped-up checks and searches.

Observers think the violence may prompt Marcos to prolong martial law. But a guerrilla source predicted: "He will fail, like the shah of Iran. He cannot go against the tide."