Philippine President Ferdinand Marcos opened the country's first national legislature in nearly six years yesterday by promising his opponents freedom "to disagree" but giving no sign on an early end to his one-man rule.

The president made several conciliatory statements toward his critics during a 90-minute address opening the interim National Assembly in Quezon City's new legislative building. The words seemed in keeping with his recent softening of the rules for amnesty for martial law violators and his decision to drop subversion and other charges against about 1,200 people in the past week.

Marcos said nothing, however, about releasing his leading political opponent, former Senator Benigno (Ninoy) Aquino, from prison after nearly six years of confinement. Opposition sources have predicted in the past week that Aquino would soon be released and go into exile in the United States.

The martial law government was "giving to hundred of detainees an opportunity to begin their lives anew under the New Society," Marcos said. "May they use this opportunity well."

In the past few months Marcos has been alternately loosening and tightening martial law restraints. He appears to be searching for a formula that will both maintain order and business confidence and appease human rights advocates at home and in the United States, Manila's most important economic and military ally.

"We shall now seek to enlarge the democratic dialogue with all sectors of the country," Marcos said. He also said, however, that "a weak government is not a safeguard but a menace to national security and human rights."

"While I am as anxious as anyone else about the lifting of martial law, prudence advises me not to speculate as to when we could finally do it," he said.

After allowing his opponents to campaign relatively freely for seats in the National Assembly early in the year. Marcos's police jailed several hundred people who protested the near-total victory by Marcos supporters and what many charged was a rigged vote-counting operation.

In the last four weeks, however, he has met privately with some of his most vehement critics. Hundreds of subversion cases have been dropped. Aquino's sister, movie and television director Lupito Concio, has been granted permission to leave the country for a Hong Kong vacation despite being named on an official travel blacklist. Nelia Sancho, a former Pacific-area beauty queen jailed for her involvement with Communist guerrillas in a celebrated case two years ago, was released late last week, opposition sources said.

This most recent period of relaxation began just after U.S. Vice President Walter Mondale visited Marcos in early May. Mondale gently warned the president of the trouble that bills for military and economic aid to Manila would have in the U.S. Congress if the postelection crackdown continued.

Yesterday Marcos only referred to the Mondale visit when he hailed the formal U.S. recognition of Philippine sovereignty over American military bases on the islands. On the 80th anniversary of the Philippines' brief independence from Spain before the United States took over the islands, Marcos said this was "the first time the Philippine people are truly sovereign over their land after about 500 years" of Spanish and American domination.

The new assembly marks Marcos' attempt to convert the national government from an American system that brought presidential elections every four years and a sharing of power between the president and the Congress until Marcos declared martial law in 1972.

The assembly operates under a British, or parliamentary, system in which Marcos rules as prime minister as long as his assembly supporters are in the majority. Only a handful of the 200 delegates are not formal members of Marcos New Society Movement, and none are outspoken opponents. Marcos also retains power to make law without assembly approval when he sees fit.

The opposition he talked about in yesterday's speech includes former senators and others like Aquino who failed to be elected to the assembly but still polled about 40 percent of the vote in Manila. They want an end to martial law, a return to a free press and a legislature with full powers.

"With respect to the opposition we have taken initiatives to remove the irritants that have strained their participation in our political life," Marcos said. "We shall strive to be one nation in which one would be free to disagree with another without undermining national unity."