In a building where Ferdinand Marcos was proclaimed president for a new term four months ago -- only to flee hours later -- 48 men and women have begun the historic task of writing a new constitution that will finally dismantle the legacy of his 20-year rule.

A Constitutional Commission appointed by President Corazon Aquino on May 25 has come under criticism from various quarters. But it appears to be getting down to business and has posted its first achievement, approving a preamble last week after two days of intense debate.

More weighty matters lie ahead, however, and contentious sessions are expected as the commissioners sort out such fundamental issues as the form of government to adopt, a definition of the national borders and a ban on foreign military bases.

The final result is expected to pare down the authoritarian powers that Marcos arrogated to himself during his two decades in power.

Among the proposals already advanced in the commission are ones that would make it easier to impeach a president and more difficult for a president to declare martial law. Other proposed provisions would restore checks and balances removed by the Marcos government.

Marcos was elected to four-year terms in 1965 and 1969 under the previous, U.S.-style 1935 constitution. He declared martial law in 1972, a year before his second and final term was due to expire, while a constitutional convention was in progress. Marcos then used his powers to remain in office and influence the drafting of another constitution, which was promulgated in 1973. Marcos ruled under martial law until 1981, when he ran for a new six-year term under a constitutional amendment allowing him to run for reelection indefinitely.

A "snap election" he called for Feb. 7 proved to be his undoing, when massive vote fraud used to keep him in office caused a backlash that eventually led to his overthrow and the installation of his challenger, Aquino.

Aquino has vowed not to interfere with the commission's task, although she appointed all of its members, based on 1,500 nominations from the public. She has, however, urged the commission to "be quick" and has set an unofficial Sept. 2 deadline for completion of a new charter so that elections can be held soon thereafter.

Commission President Cecilia Munoz-Palma, a former Supreme Court justice, said "much will depend on committee work" as the members try to put together various elements of the new constitution. She said the members were committed to completing the text by Aug. 25.

Initial progress appeared less than auspicious when commissioners became mired in debate on a preamble. After two days, a preamble was adopted that was not much different from that of the 1973 constitution, which is being used as one of the body's "working references."

But according to former labor minister Blas Ople, who represents Marcos supporters, "The preoccupation with speed and efficiency might risk stampeding members to a hasty approval." The three-month time limit, he said, "is too agonizingly short."

At present, he said, a majority of the members appear to favor a strong central government rather than a federal system, a presidential rather than a parliamentary administration, and a unicameral instead of a bicameral legislature.

Commission sources said it appears that a major influence in shaping the charter will be a coalition of clerical and lay leaders and some of the left-leaning members.

One of the most divisive issues could turn out to be a provision banning foreign military bases, the sources said. It would be aimed at getting rid of two major U.S. facilities, Subic Bay Naval Base and Clark Air Base, which have been the targets of protest by communist and other left-wing nationalist groups.

Ople said he expects a resolution to be submitted shortly demanding a ban on foreign bases. But he estimated that the leftist forces would muster no more than 20 of the 48 votes.

The left already was seen as suffering a defeat on the preamble when a bid to replace a reference to "God Almighty" with the term "lord of history" was rejected, Ople said. He described the term as a "Marxist notion."

At least one member, peasant leader Jaime Tadeo, has been identified with the National Democratic Front, an organization of the Communist Party of the Philippines. Sixteen members are said to represent religious interests identified with a wide range of political affiliations.

Another sensitive issue, on which party lines are unclear, is a definition of national borders that would solve the persistent problem of a Philippine claim to Malaysia's oil-producing state of Sabah. The dispute has stood in the way of improving relations with Malaysia and ending Sabah's role as a training and staging area for Moslem guerrillas fighting for independence in the southern Philippines.