The Philippines' scattered opposition factions succeeded in forming an alliance yesterday after eight years of divided efforts to combat martial law rule. But it was unclear whether they could force President Ferdinand Marcos to revive democratic government even if they maintained their unity.

Eight political parties formed a National Covenant for Freedom. In a meeting at a country club they adopted a plan demanding "immediate termination of the Marcos dictatorship," unconditional dismantling of marital law, convocation of free elections and the end to what was described as foreign domination.

Diplomats present at the launching questioned whether they were witnessing a major development or a bit of political theater.

Coming at a time when radical elements are setting off bombs in Manila and students and labor are protesting price increases, the rhetoric was ominous. Gerardo Roxas, president of the Liberal Party, declared: "This may well be the last effort for a peaceful solution to the problem. If our efforts should fail; I believe the resolution will rest in the hands of our people."

It took 14 revisions for the leaders to agree on points broad enough to encompass all their groups. Opposition parties in this country, one observer said, are characterized by the presence of more generals than soldiers. "Everyone want to lead," he added.

The alliance does bring together the Liberal and Nationalist parties that dominated political life before Marcos imposed martial law in 1972. Marcos was elected on the Nationalist ticket in 1965. He then used his faction to form the New Society Movement that now dominates the rubber-stamp National Assembly.

Ex-senator Benigno Acquino, who inspired creation of the National Covenant coalition, is in the United States seeking American support for himself as an alternative to Marcos -- or at least to neutralize U.S. backing of the Marcos government.

Aquino, on leave from jail for heart surgery in the United States, stayed on as a fellow at Harvard University. He will send his endorsement of the covenant, opposition leaders said.

The united opposition clearly has pro-United States leanings.

One, insider, said the opposition coups feel that as long as they remain disunited, their chances of getting the ear of the United States are slim. The United States, until 1946 the colonial power there, maintains two military bases-in the Philippines.

An opposition source said the alliance was spurred by reports that Marcos, who will be 63 in September, is in ill health. The source said, "Should anything happen to Marcos, and if the military needs to move in, it can fall back on the united opposition for political support.

Roxas said the covenant members "intend to pursue every means to achieve our objectives.It is our intention to organize in every city and town to awaken the people to the evils of this repugant regime and hopefully to bring public pressure on Marcos to dismantle martial law."

The opposition offered no program of government. Roxas promised a more comprehensive document later.

Former Nationalist Party senator Jose Diokno said: "For eight years the town main political parties machineries are in deep freeze. I can't tell exactly what strength they have now.But at least they can be heard as one voice and act together."

Much will depend on the reaction of Marcos. One observer characterized some of the 70 or so signatories of the covenant as discredited politicians who could be easily bought off by Marcos.

The government appears relaxed about the opposition move. Marcos in his own pronouncements has often urged the opposition to unite, at least to give some semblance to his contention that dissent is allowed.

A senior Cabinet minister said: "Of course, we are interested that they are uniting. But we are not intimidated."

A more immediate headache for Marcos is the festering Moslem rebellion in the southern Philippines. Before the opposition could finish reading its covenant to the press and diplomats on the need "to rectify injustices committed against our Moslem brothers," Marcos announced peace talks with the rebels would take place in Jakarta Sept. 5 and 6.

The Libyan-backed secession effort began upon declaration of martial law and has since claimed more than 50,000 casualties. It also has sapped the Philippine economy.

The meeting next Friday, arranged by the Islamic conference, will be the first attempt to find a peaceful solution to the conflict since talks collapsed in 1977 over definitions of autonomy.