DAVAO, the Philippines— A year ago in the densely populated urban shantytown of Agdao, three men gunned down a notorious Communist hit man, raised their rifles and shouted, ’’Arise, masses!’’

With this now-legendary incident was born a right-wing vigilante group whose imitators are quickly spreading through the Philippines in a new approach to counterinsurgency.

Many military and Government leaders, including President Corazon C. Aquino and Gen. Fidel V. Ramos, the Chief of Staff, have endorsed the concept of anti-Communist vigilante groups, even as fears have grown that a new form of armed terror has taken root in this violent nation.

The central Government appears to have been caught unprepared as copycat groups spring up around the country, and observations on the scene clash with military assurances that they are spontaneous and unarmed.

 Outside Influence a Question

Interviews with officials in Manila, Cebu City and Davao, of which Agdao is a part, left unanswered questions about any possible outside coordination of these groups and their new strategy.

Among other organizations, Causa, the political arm of the Unification Church founded by the Rev. Sun Myung Moon, appears to have sought to capitalize on growing right-wing sentiment in the Philippines.

The original vigilante group, which has taken its name from that cry of vengeance, Alsa Masa, is armed and controlled by the local military commander in the city of Davao, which had become known as an urban ’’laboratory’’ for the insurgents.

With a system of checkpoints, armed patrols, taxation, propaganda, forced recruitment and summary justice that consciously mimics that of its Communist enemy, Alsa Masa has largely succeeded in driving the rebels from a city where, in many areas, they had taken effective control.

The Alsa Masa leader in Agdao, a 45-year-old tire dealer named Rolando Cagay, said his group raised money with weekly benefit dances.

Mrs. Aquino’s secretary of local government, Jaime Ferrer, has ordered local officials throughout the country to form unarmed vigilante groups patterned on a village association called Nakasaka, or People United for Peace.

 Unarmed Only in Theory

Davao’s military commander, Col. Franco M. Calida, dismissed the notion that unarmed groups could succeed, saying, ’’This is an armed struggle.’’

A reporter visiting Davao del Sur, a few miles south of Davao, where Nakasaka members man checkpoints with machetes and bows and arrows, found that some of them also carried pistols and homemade pellet guns.

There is an atmosphere of surprised relief in Davao at the quick retreat of open Communist force. The level of violence has sharply dropped in a city where three or more people were killed each day, either by the insurgents or the military.

But it is not clear to what extent the vigilante group was responsible for the rebels’ retreat, or what its long-term success may be.

Colonel Calida said the withdrawal of the rebels from Agdao was a result of the success of ’’deep penetration agents’’ who touched off a violent reaction from the Communists. The rebels’ fear of informers brought a series of killings in Agdao that alienated a previously supportive population.

 ’We Became Angry at Them’

’’They were like gods here before,’’ said Alejandro Amorado, standing at an Alsa Masa checkpoint in Agdao, speaking of the Communists. ’’They killed the abusive people or drove them out of here. The people loved them.’’

’’But then they started killing even people who had not done anything – ice cream vendors, people who sold slippers,’’ he said. ’’We became angry at them.’’

A local spokesman for the National Democratic Front, an umbrella organization that represents the Communists, acknowledged that the rebels had made mistakes and that Alsa Masa had been ’’tactically effective’’ in forcing them to return underground.

But the spokesman, who called himself Ka Gary, said several the vigilante groups were what he termed ’’Alsa drama,’’ putting on a show of converting to anti-Communism while they await a backlash that could ultimately play into their hands.

Some military officers expressed private concern that ’’deep penetration’’ could work both ways and that the weapons they issued to vigilante groups could fall into the hands of the insurgents.

 ’No Way to Be Neutral’

Meanwhile, Alsa Masa and its imitators have succeeded in polarizing the population.

’’In the fight between democracy and Communism there is no way to be neutral,’’ Colonel Calida said. ’’Anybody who would not like to join Alsa Masa is a Communist.’’

Alsa Masa members interviewed at checkpoints around the city said that each home was required to contribute one member to its nightly patrols and that the houses of those not cooperating might be painted with a red ’’X.’’

The Roman Catholic Church has also been caught up in the spreading polarization. The Auxiliary Bishop of Cebu, Msgr. Manuel Salvador, said recently, ’’We really cannot blame these civilians who decide to arm themselves.’’

As he spoke, vigilante groups were listing as targets other local churchmen who have been active in human-rights work. Priests of the Redemptorist order in Davao have been labeled ’’Redempterrorists,’’ and a group of Carmelite nuns has received threatening letters.

The police commander for Davao del Sur, Col. Jesus R. Magno, called the vigilante mobilization ’’basically a psy-war operation.’’

On a local radio station, a broadcaster named Jun Porras Pala read from a right-wing Causa manifesto and threatened doubters with retribution.

’’We will exhibit your heads in the plaza,’’ he told his critics in a recent broadcast. ’’Just one order to our anti-Communist forces, your head will be cut off. Damn you, your brains will be scattered in the streets.’’

It appears that the vigilante groups have carried out several killings as well as other acts of intimidation. Colonel Calida said 11 Communist rebels had been killed by vigilantes in Davao.

A former leftist militant who is setting up an Alsa Masa branch in Cebu City, Marianito Ventura, said: ’’Of course if we come across a member of a sparrow unit, we liquidate him without notifying anybody. That is what they are doing to us.’’ Sparrow units are Communist assassination squads.

He said his organization was useful to the military as a means of avoiding investigations of human rights abuses. ’’We in the Alsa Masa don’t give a damn about a review from the top,’’ he said.

 Vigilantes in Other Regions

In recent weeks, vigilante groups have been quickly forming in other regions, adapting themselves to local conditions, and reports of abuses have begun to reach Manila.

On the sugar-growing island of Negros, planters have formed El Tigre, a network of armed groups whose threats this week drove 78 families to flee a rebel-influenced mountain area.

On the island of Cebu, a group called CACA, or Citizens Against Communism Army, has sent out armed patrols in remote areas that look in photographs like Communist rebel patrols and are said to have carried out attacks similar to rebel attacks.

In some areas of Mindanao and Cebu, right-wing groups have linked up with machete-wielding religious cultists called Tadtad, or Chop Chop, who believe they are immune to bullets and who ritually mutilate their victims.

Elsewhere, renegade military officers implicated in recent coup threats are said to have formed armed groups that advertise themselves as anti-Communist squads.

 Political Coercion Feared

Opponents of the vigilante groups warn that they provide the makings of private armies for a new generation of the warlords who have exercised local power in the Philippines.

’’In a country with a very recent history of warlordism and fascism and right-wing military activism, I think this is a very dangerous phenomenon,’’ said a senatorial candidate from Cebu, John Osmena. ’’These groups can become the new instruments of political coercion.’’

Already, the rapid growth of Alsa Masa throughout Davao has affected the tenor of local politics.

Few politicans openly criticize Alsa Masa, and even Mayor Zafiro Respicio, a human-rights exponent in the past, has found it expedient to endorse vigilantism., despite privateley expressed doubts.

’’The whole city has turned right, hard right,’’ said the managing editor of a local newspaper, Alberto Tesoro.

On a visit to Davao recently, President Aquino endorsed unarmed citizens’ groups as a ’’concrete manifestation of people power and an effective weapon against Communism without the use of firearms.’’

But a political science professor at the University of the Philippines, Francisco Nemenzo, called this notion ’’a mockery of the concept of people power,’’ the term Mrs. Aquino gave last year to the peaceful protests that helped force President Ferdinand E. Marcos from power.

SETH MYDANS

Correction Appended

---