President Ferdinand Marcos and his supporters appear to be broadening their suppression of dissent, turning increasingly to lengthy court proceedings against journalists, priests and even elected local officials.

In recent weeks authorities have arrested three priests, including an Australian and an Irishman, the head of a newspaper workers' union, the mayor of a southern provincial city, his deputy and five city councilmen. In addition, arrest warrants on criminal libel charges have been issued for the editor and a Manila staff member of the Far East Economic Review, a regional news magazine based in Hong Kong, but the documents have yet to be served.

Marcos has decreed that antigovernment agitators and publishers who allow their facilities to print propaganda may be executed if convicted, according to documents quoted by United Press International Sunday.

The documents also authorize trials in absentia of Marcos' political opponents and authorize the government to confiscate their property in the Philippines upon conviction. The penalties were included in two presidential decrees allegedly issued two years ago but released only last Tuesday to the Philippine Supreme Court.

Presidential Decrees 1834 and 1835 were purportedly signed by Marcos Jan. 16, 1981--a day before he lifted eight years of martial law.

To date, all those arrested have been released on bail except Aquilino Pimentel Jr., the popular mayor of Cagayan de Oro city on the troubled southern island of Mindanao. But opposition leaders fear the government may be able to stifle political opposition through protracted legal proceedings linking the defendants with subversion and rebellion cases already under way.

The arrests come at a time of mounting insurgency by guerrillas of the Communist New People's Army, especially in Mindanao. Some political observers believe Pimentel, a rising star in opposition ranks, is being made a scapegoat for the military's failure to put down the rebels.

Marcos ordered Pimentel's arrest April 17 on rebellion charges after a purported former New People's Army leader, Carlito Sandag, alias "Commander Delmo," testified against the 43-year-old mayor. Sandag contended that Pimentel once gave him 100 pesos (about $11) and told him to "keep up the good work" in trying to overthrow the government.

In a telephone interview from a military camp in Cebu where he is detained, Pimentel said he did not remember having met anyone by that name. "If I gave 100 pesos to him," he added, "it was not for a revolution." Of the congratulations attributed to him he said, "That's ridiculous. I certainly deny having said that to anyone."

"I feel this is a political matter," he said, "principally because I've been very active in organizing at the grass-roots level of the PDP," the opposition Philippine Democratic Party.

He said his party, which has a "socialist orientation" and has allied itself with the Laban Party, had been "growing by leaps and bounds." He said his arrest under a presidential decree that allows indefinite detention without bail makes it "much more difficult for the party to grow and organize," but "the setback will only be temporary."

Pimentel said the New People's Army was "certainly gaining ground" in his province of Misamis Oriental, and blamed the rebel gains on "injustices by the regime in trying to apply a military solution to a nonmilitary problem" and "the influx of multinationals at the expense of the indigenous population."

In the latest series of arrests, the vice mayor of Cagayan de Oro and five city councilmen were picked up by paramilitary Philippine Constabulary officers Tuesday and charged with "indirect contempt of court" in the case of Pimentel.

The charge stemmed from a city council resolution calling for Pimentel's release and the granting of permission for a demonstration supporting him. The Philippine armed forces' advocate general, Brig. Gen. Hamilton Dimaya, requested the arrests and called for the officials' suspension from public office for the duration of the Pimentel case.

According to western diplomats and political observers, the case illustrates political polarization as well as Marcos' extraordinary powers.

"My view is that the definition of subversion is creeping further along the spectrum of opposition, and this is an instance of it," said one senior western diplomat.

"A lot of government people consider it an unfortunate political move," said another. "For most of the last six months the government has been saying over and over it is anxious to develop the legal political opposition and hopes it will participate in elections next year.

"This kind of thing turns the opposition sour" on elections and could fuel arguments for another boycott, as in the June 1981 presidential balloting, he said.

So far, however, the opposition has reacted by intensifying its efforts to develop a credible coalition, uniting Pimentel's Pilipino Democratic Party-Laban group with the United Nationalist Democratic Organization known as Unido.

Pimentel's group has not even joined in Unido's demand for major electoral reforms as a precondition for participating in May 1984 parliamentary elections, insisting simply that "only live voters vote and all the votes be counted."

Opposition leaders plan to announce the coalition next month. They say it will have a more radical platform than Unido has previously espoused.

This exemplifies a shift of moderate opponents to the left as Marcos continues to wield powers of martial law in practice if not in name.

One key decree issued in May 1981 allows him to order the summary arrest and indefinite detention of suspected subversives. Release can be granted only by another presidential order. Opposition lawyers say hundreds of persons besides Pimentel have been arrested on such "presidential commitment orders."

The Supreme Court recently turned down an opposition challenge to the legality of these presidential orders.

One lawyer, Joker Arroyo, said that under the presidential order, Pimentel "can rot in jail indefinitely" despite having been elected in 1980 to a six-year mayoral term.

Now Arroyo and other defense lawyers are battling what they say are the military's highly irregular efforts to insert Pimentel in an ongoing case in Cebu of alleged rebellion. The charges against Pimentel currently carry a penalty of 12 to 20 years in prison, but if he is linked to the Cebu case, which involves a murder charge, the maximum penalty would be death.