President Reagan said last night that he seeks removal of the leftist Sandinista regime in Nicaragua unless it changes its goals and allows the "freedom fighters" militarily opposing it into the revolutionary government.

At a nationally televised news conference, the 28th of his administration and first of his second term, Reagan took a militant tone in opposing the Sandinistas and went further than he has previously in calling for a new government in Nicaragua.

Asked whether he sought removal of the Sandinista regime, Reagan replied: "Well, removed in the sense of its present structure, in which it is a communist totalitarian state, and it is not a government chosen by the people."

The president held his ground in response to follow-up questions on the same theme. He said the "freedom fighters" opposing the Sandinistas had been a part of the revolution that the current government had betrayed with a "brutal" suppression of civil liberties.

When Reagan was then asked whether he was advocating the overthrow of the government, he said, "not if the present government would turn around and say, all right, if they'd say 'uncle.' "

Reagan also appealed for congressional support for the rebels, who are known as "contras" but usually referred to as "freedom fighters" by the president. In a radio speech Saturday from his California ranch Reagan called the rebels "our brothers," and a spokesman last night said he was mounting an intense "educational" campaign to persuade Congress to free $14 million in funds for the rebels.

Last night, when Reagan was asked about the prohibition on aid to the rebels that Congress imposed five months ago, he treated the question as if it concerned a political proposal rather than current law.

Asked specifically about the Boland Amendment, which prohibits aid to the rebels, Reagan said: "I think that some of the proposals that have been made in Congress have lacked a complete understanding of what we're trying to do."

Reagan's emotional denunciation of the Sandinistas in his radio speech was reinforced by his statements last night in the news conference in the East Room. He defended the rebels as genuine revolutionaries against the regime of the late Nicaraguan dictator Anastasio Somoza. He denounced what the Sandinistas have done as "totalitarian . . . brutal, cruel" and said that U.S. assistance is consistent with the charters of the United Nations and the Organization of American States.

"We're saying we're trying to give those who fought a revolution to escape a dictatorship, to have democracy, and then had it taken away from them by some of their fellow revolutionaries -- we're saying we want them to have a chance to have that democracy they fought for," Reagan said. "And I don't think the Sandinistas have a decent leg to stand on."

Immediately after the news conference, the Nicaraguan Embassy issued a statement saying Reagan's assertion "that the contra revolutionaries were part of the effort to overthrow the Somoza dictatorship is incorrect. The leaders of the contra revolutionaries fought to that end to sustain the Somoza dictatorship in Nicaragua. When they failed in that effort, they fled Nicaragua and . . . financed, organized and trained by the U.S. CIA, they are attempting to restore the same oppressive regime that existed before the triumph of 1979."

In contrast to the strong language Reagan used about Nicaragua, he appeared to be deliberately careful in his references to the Soviet Union.

Asked how far he was prepared to go in making concessions to get an arms control agreement at the negotiations that are to begin in Geneva next month, he expressed optimism that the Soviets will "stay at the table and negotiate with us." At the same time, the president showed no sign of yielding on his Strategic Defense Initiative, called "Star Wars" by its critics.

The Soviets have indicated that U.S. concessions on space defense are essential to successful negotiations in Geneva. Reagan repeated his oft-stated view that the United States simply is engaging in research permitted under the 1972 Anti-Ballistic Missile treaty and is not seeking deployment.

At his last news conference on Jan. 9, Reagan said that if research produced a deployable defense "we would be willing to go into negotiations and discussions with the other nations of the world and with our allies about what to do about that and whether and how to deploy."

Because any deployable defense is believed to be at least a decade away, however, any such decision would be left to a future president and Congress.

Reagan last night repeated, in mild language, his claim that the Soviets have violated the unratified Salt II treaty while the United States has abided by its provisions. But he said the United States would not do so indefinitely.

"We'll have a decision several months from now to make with regard to whether we join them in violating the restraints," he said.

As an example, Reagan said the United States is approaching the treaty limit of 1,200 long-range nuclear-weapon launchers. Officials previously have said the administration will exceed that limit when a new Trident missile-firing submarine, the Alaska, joins the Navy, unless older subs are retired.

Reagan said that when the Soviets have faced such choices they have retired their subs but converted them into submarines for firing cruise missiles.

He said he "didn't recall" anything of the kind reported by columnist Jack Anderson, who said Reagan had passed word to Moscow in 1982 that he would not sign Salt II if it were ratified and that the Soviets in 1982 then told him they would not be bound by the treaty.

"I went into the office and I said, 'Where is all this coming from?' I do not remember any statement from the Soviet Union of that kind," Reagan said.

On another question involving U.S.-Soviet relations, the Vienna talks held by the two nations this week on the Mideast, he described the discussions as an opportunity to exchange views "and make sure that there couldn't be ny miscalculations that could lead to some kind of confrontation or problem."

Asked whether the Soviets had requested a direct negotiating role in Mideast diplomacy, Reagan said, "I haven't had a full enough report to say whether they mentioned some specific things."

The president also gave a general answer to a question about whether he saw "weakening signs" of commitment in Greece and other nations on the southern flank of the North Atlantic Treaty Organization.

Without being specific, Reagan said, "we're very concerned about some of the bilateral problems between countries there at our southern flank of NATO and the effect that they can have on the whole security of the alliance."

Though he sometimes seemed less assured than at his last news conference, Reagan appeared combative and engaged whenever the Central American issue was raised.

"We believe . . . that we have an obligation to be of help where we can to freedom fighters and lovers of freedom and democracy, from Afghanistan to Nicaragua . . . ," Reagan said.

The president's radio speech and his repeated statements last night indicated a new, more militant mood on obtaining aid for the anti-Sandinista rebels, which administration officials have described as an uphill fight in Congress.

But Reagan has vowed to take his case for aid to the people. Last night was a step in that endeavor.