President Raul Alfonsin predicted today that Argentina will face "a hard period" of social and political unrest in the coming months as his government implements economic austerity policies and prosecutes former military leaders for human rights crimes.

On the diplomatic front, Alfonsin termed President Reagan's latest peace proposal on Nicaragua a "positive policy" that could provide a basis for a solution to conflict there.

Alfonsin said that he had decided to take "strong measures" to control inflation "that is reaching dangerous levels," and that Argentines would have to make "the necessary effort" to stabilize the economy.

At the same time, he reiterated his determination to support the public trials of nine former military commanders, scheduled to begin later this month, despite efforts to "create tension" by "one extreme or the other."

These two policies, he said, meant "we are going to live through a few months of protests, of demands, of political and social tensions. Adding up the economic, social and political problems, we are going to pass through a really hard period."

Alfonsin said that he believed the new democracy "will not be placed in danger" by the expected troubles. He added, however, that his administration could not "conspire against the system" with too much austerity and that he would continue to avoid policies in which "the price has to be paid by the workers."

The president's comments came in an hour-long interview at the government palace here with Katharine Graham, chairman of the board of The Washington Post Co., and a group of editors and reporters.

The statements reflected the 16-month-old government's concern over inflation that has risen from an annual rate of 450 percent to more than 800 percent and Alfonsin's apparent willingness to accept the political costs of stabilization measures he once avoided.

"Our principal concern has been that the hardship not fall on the backs of those who have the least," said the president, who sought unsuccessfully last year to control inflation even while raising some workers' wages. "But it is clear that we are not going to be able to offer the improvements we would like in order to fullfil the requirements of social justice."

Alfonsin, 58, conceded that salaries had recently dropped because of the government's measures. However, he stressed that he intended to prevent wages from falling below their present levels this year, despite pressures for further reductions in government spending from the International Monetary Fund.

The IMF suspended its economic agreement with Argentina last month because of its failure to meet targets for reducing government spending and monetary growth in fighting inflation. Argentina must now win approval from the IMF of a new economic plan to preserve its agreements with foreign banks for financing its $48 billion foreign debt.

Alfonsin conceded that "inflation did not yield" under Argentina's application of the IMF-approved program last September. Continuing price rises, Alfonsin said, only demonstrated that "the problem of inflation cannot be resolved in two or three months even if hard measures are taken."

The IMF, he said, "should not address nominal guidelines as much as the effort we are making and the mechanism we have put into place. What we would like is that these guidelines not exist once the program is defined."

The government's efforts to control inflation and negotiate with the IMF will coincide with the long-delayed trials of the nine military commanders, who served in the three juntas of Army, Navy and Air Force leaders who ruled Argentina from March 1976 until June 1982.

The commanders, including former military presidents Jorge Videla, Roberto Viola and Leopoldo Galtieri, are charged with responsibility for thousands of abductions, killings and cases of torture attributed to military forces. Their trials, initiated with government charges shortly after Alfonsin's inauguration, were transferred from military to civilian judges after the armed forces' supreme council declined to act.

Alfonsin said he was "bothered" by the frustration of his plan to have the military judge its own alleged offenses and the consequent long delay in the resolution of the cases. He said "it would have been much better for the country" if the military had acted.

The president said he thought the public trials, now expected by court sources to begin April 22, would provoke resistance. Some sectors, he said, "pretend that this trial is an attack against the prestige of the armed forces."

These opponents, he said, "I suppose will be active and will create tensions." However, he said, "90 percent of the Argentine people support the policy of the government. We get criticized by one or the other extreme."

Commenting on U.S. policy in Latin America, Alfonsin said he believed there had been "an important change" of policy in President Reagan's recent peace proposal to Nicaragua's leftist government. "I think it is a positive policy that, if taken up by Latin America, might produce some formula for a solution," he said.

Alfonsin said "it was very clear" that the Reagan administration had wanted to overthrow Nicaragua's government. "Now there's another language, another proposal," he said. "In my judgment, this allows the buying of time so that in Latin America an idea can be developed that is compatible with nonintervention."

Describing some aspects of Reagan's plan as "much too severe," he welcomed "a substantial change from a military solution to a political one." Alfonsin visited Reagan last month. Press Secretary Jose I. Lopez said the United States sent a written version of the plan, which seeks talks between Nicaragua and U.S.-backed rebels, and that Reagan called Alfonsin Monday to discuss it.

Lopez said Nicaraguan President Daniel Ortega called today. Nicaragua has rejected Reagan's initiative.