UNITED NATIONS, OCT. 7 -- The U.N. General Assembly expressed by acclamation its "firmest support" today for the peace agreement signed two months ago by five Central American nations.

The resolution also provided a mandate for U.N. Secretary General Javier Perez de Cuellar to play a major role in monitoring the treaty commitments when they come into force one month from today.

A U.N. role could range from the dispatch of a team of observers to the creation of a full-fledged peace force, said a ranking U.N. official. He noted that under the agreement, the secretary general is a member of the "verification commission," but thus far its specific functions have not been spelled out.

The resolution was sponsored by the five signatories of the agreement -- Costa Rica, El Salvador, Guatemala, Honduras and Nicaragua -- and eight Latin American nations that have backed the peace process.

They said the resolution was timed to precede President Reagan's speech on the peace process before the Organization of American States in Washington today, and Thursday's General Assembly speech by Nicaraguan President Daniel Ortega.

"We wanted to put it on record so neither side could claim credit for it or torpedo it," said a diplomat from one sponsoring nation. To beat these deadlines, they violated the normal U.N. practice of taking up substantive issues only after the assembly hears policy statements by 100 or so foreign ministers.

U.S. representative Alberto Martinez Piedra repeated guidelines the White House has offered the Nicaraguans and proposed a "lead" role for the OAS, rather than the United Nations in monitoring the agreement.