President Carter ordered a 25 per cent temporary reduction in federal hiring yesterday pending the development of new ceilings on the number of federal employees.

In another development, the President took a further step in his effort to encourage public participation in government decision making, inviting 300,000 randomly selected Americans to help him devise a national energy policy.

The hiring reduction was contained in a memorandum from the White House to all executive branch departments and agencies. In it, Carter ordered that no more than 75 per cent of the vacancies that occur in the federal civilian work force after Feb. 28 be filled.

The job reduction order will remain in effect at least until April, when the Office of Management and Budget is to issue new ceilings on the number of civilian workers in federal agencies.

While not committing the administration to "an absolute reduction in the number of employees," White House secretary Jody Powell said the President issued the order to "put the departments in a better position to deal with the new job ceilings," which presumably will be lower than existing ceilings.

As a temporary measure, Carter's order is likely to have little impact on the size of the federal work force. An estimated 15,000 federal jobs become vacant each month. Thus, the President's directive would prevent the filling of about 7,500 jobs over the next two months.

Moreover, federal employment has not been growing in recent years. The number of federal employees is about the same today as it was 10 years ago, while the percentage of federal employees in the total population and among all government workers in the country has actually been decining for several years.

Carter's directive also contains a number of exceptions to the hiring reduction, among them the political appointments his administration will make to policy-making jobs in the administration.

As of Dec. 31, the federal civilian work force numbered 1.9 million, exclusive of Postal Service employees. The existing ceilings, developed by the Ford administration, are actually estimates of the maximum number of federal employees and are not ceilings in the sense that they are legally binding.

Those estimates anticipated a federal work force of 1.951 million by next Sept. 30 and of 1.954 million by Sept. 30, 1978.

On the energy policy letters, Powell said the White House had no cost estimate for the project, in which 300,000 people selected at random from Census Bureau lists will be invited to comment. However, postage alone would cost $39,000.

The White House is sending the same invitation to another 150,000 people who are state and local government officials and members of conservation, public policy, business and other groups. That mailing will cost another $19,500 for postage alone.

Powell scolded White House reporters on the subject, saying "the people in this room seem the least interested in public involvement in the government process." He said White House reporters have displayed "a degree of cynicism that I have found surprising." Powell later apologized, calling his remark "inappropriate."

The 450,000 letters ask for suggestions on such issues as energy conservation, development of new energy resources, environmental protection and the sharing of burdens imposed by the energy shortage.

The White House asked that the comments be sent to Box 2778, Washington, D.C. 20013, by March 21.

The President yesterday also attended the swearin-in ceremonies pointees - Max Cleland, administrator of the Veterans Administration, Sam Brown, the head of the federal volunteer agency Action, and Evan Dobbelle, U.S. chief of protocol.

Carter noted the "striking contrast" between Brown, a one time anti-Vietnam war activist, and Cleland, who lost both of his legs and one arm from wounds he suffered in Vietnam.