President Reagan yesterday said he would appeal for nuclear disarmament in a speech to the United Nations in June and called on Soviet President Leonid I. Brezhnev to meet him there to discuss arms control.

But at the same time that Reagan was continuing the arms reduction overtures he launched last week in a prime-time news conference, he again condemned the Soviet Union, which he said had engaged in "crimes" against innocent people in Afghanistan.

Reagan also repeated his assertion of last week that the Soviets have a nuclear arms margin of superiority over the United States, a view disputed by many experts.

The president's disclosure that he would address the United Nations came in the Oval Office at a carefully planned question-and-answer session that is part of a White House communications format designed to dominate the news with presidential proposals. In a sharp departure from procedure, Reagan said he plans to hold similar meetings with the press each week.

Reagan plans to address a special U.N. session on disarmament in mid-June after he returns from a European trip, where nuclear weapons are certain to be an important item on the agenda.

"And I hope, very much, that President Brezhnev will be on hand to do the same thing and address the same group," Reagan said. "I think that this whole idea that I have been talking about since back in the campaign of arms reduction, arms control, is one of the most important things that is facing us . . . . "

The White House, however, has issued no formal invitation to Brezhnev, who is reportedly in poor health. Last week, Reagan opened his news conference by inviting the Soviets to join the United States in negotiations intended to reduce the nuclear arsenals of both countries.

At the same time, the president said a nuclear freeze would be "disadvantageous--in fact even dangerous" to the United States because the Soviets have a "definite margin of superiority" in nuclear arms.

He also contradicted conventional wisdom among nuclear arms analysts by asserting that the Soviets have a second-strike capability that could absorb a U.S. retaliatory strike.

Reagan stuck to his ground when challenged yesterday on these assertions. He also responded to charges that he should not have said what he said, even if it is true, because it conveyed an impression of U.S. weakness.

"No, I am not sorry I said it because . . . we know for sure the Russians know that," Reagan said. "I think the American people ought to be able to know everything they know."

After asserting again that the Soviets have second-strike capability, Reagan added: "The idea is that we must have a deterrent. Our goal is peace. And to have peace, we must have a deterrent that would prevent someone from adventuring in the world using nuclear weapons."

Reagan's intent in such declarations, according to White House officials, is to win at least the war of words with the Soviets, who have made numerous arms control proposals. The president believes the Soviets take this stance because they are ahead in the arms race.

But there is no indication that the United States anticipates any immediate serious strategic arms talks with the Soviets. When a reporter noted that Reagan yesterday was, in effect, proposing a "summit" with Brezhnev, Reagan said he would not encourage the "imagery" of "a full-blown summit conference."

Reagan and White House officials declined to address speculation that Brezhnev would be physically unable to address the United Nations or meet with Reagan in June. But it is expected that the same offer would be made to anyone who might replace Brezhnev.

Soon after his meeting with reporters in the White House, the president made a blistering attack on the Soviets in a speech to building trades workers at the Washington Hilton.

"We will not remain silent when, in Afghanistan, yellow rain is dropped on innocent people, solemn agreements are flagrantly broken and Soviet helicopters drop thousands of 'butterfly' mines which maim and blind Afghan children who pick them up thinking they are toys," Reagan said. "We will condemn these crimes and work for international repudiation."

In the same speech, Reagan asserted that reducing his proposed arms budget would send "a dangerous signal" to the Soviets.

"Every penny we spend is for one sacred purpose: to prevent that first shot from being fired, to prevent young Americans from dying in battle," Reagan said.

The meeting with reporters in the Oval Office was an attempt by White House officials to change the way Reagan communicates with and through the press. Leading presidential aides have long been unhappy with the daily "photo opportunities" at which television reporters ask Reagan questions on a catch-as-catch-can basis.

On several occasions, Reagan has responded with observations that have appeared uninformed or have contradicted the impression the administration was trying to convey on a particular issue.

Under the format announced yesterday, the president will answer no questions at the photo sessions. When a television reporter noted aloud that questions could be asked even if Reagan chose not to respond to them, the president replied with a smile:

"Okay, you can, but I can sit there with a bar of soap, a pan of water in my hand ready to wash anyone's mouth out with soap."