When the first bombs went off, late in August, the initial reaction was one of bewilderment rather than panic.

Filipinos may occasionally shoot it out at election time, people said, but they don't set off bombs that could hurt innocent people. And there were jokes that the government and the frustrated opposition were carrying things too far.

Jokes like this: "It's a terrible thing," a government official says to the opposition leader. "We've counted 20 of these nefarious groups throwing bombs."

"We know about only 12 of them," the opposition leader replies, "so the other eight must be yours."

Then came a department store bombing in which a woman shopper was killed and the jokes dried up. Authorities now believe that they are contending with determined, although amateurish, terrorists hoping to destabilize the government.

Since Aug. 22, metropolitan Manila has been hit by five bombings. Banks, corporation offices, hotels and government buildings are the main targets. One person has died and more than 40 have been injured.

The bombs have sent chills throughout this sprawling capital. Many parents have kept their children out of schools. Police randomly check cars at checkpoints late at night. Security guards open packages and briefcases at hotels and office buildings. Some tours and conventions have canceled out of hotels.

President Ferdinand Marcos has said investigators have a good idea who the bombers are, but although about a dozen suspects have been arrested no one has been charged. Police say they want to make more arrests before opening their case of evidence.

A group called the April 6 Movement, named for a big 1978 anti-Marcos demonstration, has claimed responsibility for most of the blasts. Its leaflets denounce his government as corrupt and insist that only violence will bring it down. They call the bombings only "warnings" of worse things -- such as assassinations -- to come.

Authorities are known to be concentrating on a loosely affiliated group of activists who formerly opposed violence but who now have embraced terrorism as the only way of replacing Marcos.

Deputy Defense Minister Carmelo Barbero said investigators have linked the group to another bombing last December by a splinter group known as Light a Fire, whose leaders -- two of them Manila businessmen -- have admitted setting off a blast in hopes of inspiring an anti-Marcos movement.

Some in the group of suspects, Barbero said, have strong ties to the Philippine Roman Catholic Church, through relatives who are priests. Their goal, he said, seems to be to frighten but not to kill or harm. Most of the bombings have been of empty buildings, and the movement's leaflets say advance warnings were given in each case.

"They're trying to create panic and chaos and to generate nonconfidence in President Marcos," Barbero said. "They want to show that martial law does not stop violence."

Other sources, hostile to the Marcos government and having personal knowledge of the movement, said in interviews this week that they believe several different small groups are involved. They described the individual leaders as well-educated, middle-class persons, many with church connections through relatives or lay groups.

One of the self-styled "urban guerrillas" affiliated with the movement said he considered the use of force in extreme circumstances to be justified by Christian theology.

The Philippine church increasingly has been split over Marcos' rule, with some activist priests contending that overt resistance was justified and some urging the church to sanction violence. After a lengthy debate last year, the church conceded that violence might be justified in some circumstances but not in the case of the Philippines now.

The sources said the new movement is avowedly anticommunist and has no connections with the Maoist Communist Party's small armed force, the New People's Army. Leaflets of the April 6 Movement support that view. They purport to represent a third force that is the only alternative to a communist takeover once Marcos falls.

The movement also has proclaimed its choice of a new leader to be Benigno Aquino Jr., the long-imprisoned opposition leader who was released last spring and permitted to go to the United States, where he is now engaged in an anti-Marcos campaign.

Its spokesmen deny that Aquino has instigated the bombings from the United States, although Marcos has attempted to implicate his longtime foe. Police also are acting on the assumption that Aquino triggered the latest wave of bombings, according to Barbero. He points to an August speech in New York, where Aquino warned that his country would face "massive urban guerrilla warfare." The bombings began soon afterward.

Sources who have talked with the movement's leaders describe their current demands as moderate, aimed primarily at forcing Marcos to lift the martial law he imposed in 1972. "They would stop the bombings now if martial law is lifted," one of the sources asserted.

Marcos frequently has talked of eventually lifting martial law and most recently has said it might be lifted next March.