President Carter, rambling down the main street of his boyhood hometown, strongly suggested today that the national energy policy he will propose by April 20 will contain mandatory conservation measures.

At an impromptu news conference outside the aging railroad depot that once served as his campaign headquarters, the President said that overcoming the nation's energy problems will require a "combination of voluntary and other conservation measures."

"The key to the whole energy policy will be strict conservation," he said. ". . . In every possible way, we're going to save energy."

Carter mentioned only one specific mandatory provision - a requirement that automobiles operate at certain minimum miles per gallon of gasoline. Such a provision is already a part of federal law. Enacted as part of the Energy Act of 1975, it requires that 1978 model automobiles operate at an average of 18 miles per gallon. The requirement rises to 19 miles per gallon 1979 model cars and 20 miles per gallon for 1980 model cars and 27.5 miles per gallon for 1985 model cars.

The President provided no more details about a national energy policy. Asked if it would require gasoline rationing, he replied, "I don't know yet."

Three weeks after his inauguration, Carter returned to his home here Friday for what White House officials promised would be a quiet weekend of rest and study. The President, they said would conduct no public business while in Plains and would not comment on anything short of a National Emergency.

But shortly after 7 a.m. today, Carter left his home here for what turned out to be an almost three-hour moving press conference up and down Plains' Main Street.

Dressed in blue jeans, a blue shirt and gray cardigan sweater, the President was driven to the Carter family's peanut warehouse where he met privately for 45 minutes with his brother, Billy.

From there, Carter began his wandering trek up Main Street that finally ended at the railroad depot. He popped in and out of stores along Main Street shaking hands with tourists and old friends and chatting at length with his cousin, Hugh Carter Sr., and his 88-year-old uncle, Alton Carter, in Hugh Carter's antiques store.

In the course of his morning wanderings, the President:

Praised Prime Minister Indira Gandhi of India for making "some very good moves in recent weeks" toward the "democratization" of the Indian government. Carter's mother, Lillian, is in India, where she served in the Peace Corps, to attend the funeral of Indian President Fakhruddin Ali Ahemad.

Said the key element in relations between the United States and Cuba is the issue of human rights. He said he hoped a recent statement by Cuban Prime Minister Fidel Castro indicating a desire for improved relations "can be followed up by mutual efforts to alleviatc tensions and reduce animosities."

Said he had completed work on revisions of the defense budget for the fiscal year that begins Oct. 1. The revisions would reduce defense spending below the $112.3 billion proposed by President Ford, but would remain a "substantial increase" over the $100.1 billion in defense spending for the current fiscal year, he said.

Announcement that former Secre of State Henry A. Kissinger will be the chairman of a special advisory committee that will work with the National Alliance to Save Energy that was formed this week.

Said the unannounced military tests he intends to order to judge military preparedness will not be so large that they would "cause international concern." Should he order a major military maneuver as a test, he said, he would notify the Soviet Union in advance.

It was an extraordinary morning in his hamlet of 683 persons and an example of Carter's unpredictability in pursuit of what had been his normal way of life before he became President.

At Hugh Carter's antiques shop, the President chatted easily about subjects that ranged from White House newspaper subscriptions to the need for caution in speaking that is imposed by the presidency.

Hugh Carter's son, Hugh Carter Jr., is a special assistant to the President for administration, and has been in charge of slashing the frills and privileges traditionally enjoyed at the White House. Among Hugh Carter Jr.'s recent moves, the President said, was the cancelation of a number of White House newspaper subscriptions that will save an estimated $40,000 a year.

Carter, who was driven the 70 miles here frin Warner Robins Air Force Base near Macon Friday, said he had always assumed it was cheaper for him to travel by car rather than helicopter. But it turns out, he added, that auto travel is more expensive because of the number of police required to seal off intersections as the presidential motorcade passes by. So, he said, henceforth he will fly here from Macon aboard a helicopter.

Carter's comments on energy left numerous questions unanswered. He has already hinted that he will call for mandatory conservation measures, suggesting, for example, that his national energy policy will be controversial and will spark an intense national debate.

But his comments today appeared to point even more strongly toward a mandatory conservation program. Noting that the new Alliance to Save Energyt will be organized in all 50 states, the President said, "We really mean business about saving energy. We've get to let the nation know how much we waste."

On the budget revisions he will propose, Carter said he has been able to make only "superficial changes" in the Ford administration's budget recommendations. He said two problems have been to hold down the growth in the number of federal employees and to implement his proposed $31.2 billion economic stimulus package without creating a new bureaucracy.

The President said he expects to submit his budget revisions to Congress Feb. 22.