Cancellation of three-fourths of 1988 construction projects and closing of some post offices for at least the next two Saturdays were announced yesterday by Postmaster General Preston R. Tisch.

Tisch said in a statement that the cutbacks were required by the newly approved federal budget-cutting legislation, which calls for a $1.25 billion reduction in postal spending.

In response to the budget cuts, the agency will cancel 50 percent of construction projects planned for the next two years, including about 75 percent of those scheduled for 1988, he said.

Postal officials said managers are reviewing a list of several hundred construction projects to determine where cuts will be made and will announce those decisions locally.

Tisch said selected post offices nationwide will be closed Dec. 26 and Jan. 2. Decisions on which will be closed will be made by division managers, he said, and announced as soon as possible. Saturday deliveries were not canceled, however, as such a move would require permission from Congress.

Other cutbacks in service are considered likely, Tisch said, with all operating budgets under review. He said transportation for some third- and fourth-class mail will be consolidated, which should save money but could slow these classes of mail by a day or so. Third-class mail is generally advertising material, while fourth-class is parcels.