President Carter accused the government of Iran last night of an unprecedented violation of international law and standards of civilized behavior, and declared that the United States "will not rest" until the 49 American hostages in Tehran are freed "from their imprisonment and abuse."

In an opening statement at a nationally televised news conference, the president repeated the warnings the United States has made since the Nov. 4 takeover of the American Embassy in Tehran and charged that the hostages are being held in "inhuman and degrading conditions."

"Any claims raised by Iran will ring hollow while innocent people are bound, abused and threatened," he said.

The president also clearly sought to undermine Ayatollah Ruhollah Khomeini's claim to religious leadership and to isolate him from religious and potitical leaders in other Moslem nations. In an addition to his prepared opening statement, he said:

"No religious faith on earth condones kidnaping. No religious faith condones blackmail. And certainly there is no religious faith that condones the sustained abuse of innocent people."

Carter's statement, carefully crafted, reiterated the major points of administration policy since the embassy was overrun by militant Iranians with the support of that country's revolutionary leader, Khomeini.

These points included a declartion of American resolve to gain release of the hostages, a warning to Iran of "grave consequences" it faces should any Americans be harmed and an appeal to the American people to remain united and learn from the crisis "that it is our entire nation that is vulnerable to being held hostage" by dependence on foreign oil.

Administration officials have made clear that the "grave consequences" Iran faces should the hostages be harmed are military action by the United States. In virtually the same language used when the threat of military retaliation was first raised last week, the president repeated the threat last night.

He said he hoped that United Nations Security Council debate Saturday night on the crisis will lead to "a peaceful solution, because a peaceful solution is preferable to the other remedies available -- for the United States and the world.

"At the same time we pursue such a solution with grim determination," he added, "the government of Iran must recognize the gravity of the situation it has created itself -- and the grave consequences which will result if harm comes to any of the hostages."

By referring to the scheduled U.N. debate, Carter suggested no unilateral U.S. action before then. By early next week also, Iranians will have voted on a new revolutionary constitution that will ratify Khomeini's power. And Iran's deposed ruler, Shah Mohammad Reza Pahlavi, may have returned to Mexico from the New York hospital where he is being treated for cancer.

The president, however, offered no glimmer of hope for a solution in the near future, and suggested that the situation could worsen.

"In these days to come, our determination may be even more sorely tried," he said. "We will continue to defend the security, honor and freedom of Americans everywhere. We will not yield to blackmail."

While most of what Carter said last night was reiteration of previous administration statements, he placed special emphasis on treatment of the hostages. Through much of the crisis, administration officials had said there was no evidence of physical mistreatment of the Americans. But on Tuesday, Carter told a group of visitors to the White House that the hostages had been abused, a theme he repeated last night.

The news conference, Carter's first since the Nov. 4 takeover of the embassy in Tehram, was totally dominated by the Iranian crisis. All 13 questions during the 30-minute session dealt with Iran.

And the president, while seeking to rally American and world opinion to his side and underscore his warnings to Iranian authorities, was clearly constrained in what he could say. His painful situation was illustrated when he was asked how long the United States could allow the hostages to be held before inflicting on Iran the "grave consequences" he has treatended.

He replied that the fate of the hostages is constantly on his mind. "I never forget for one moment that I'm awake about the hostages whose lives basically depend on me," he said.

In these circumstances, Carter said, it is neither possible nor avisable to set a dedline for action by the United States, for to do so while diplomatic efforts continue could lead to the deaths of the hostages.

In response to questions, the president strongly defended his decision to allow the shah to enter the United States. The decision was made "to save his life," Carter said, and he reached the decision "without pressure from anyone."

Asked what role former secretary of state Henry A. Kissinger played in the decision, Carter replied: "None. I did not hear at all from former secretary of state Kissinger nor did he contact Secretary of State Vance during the days we were deciding the shah should come to the United States for medical treatment to save his life."

Kissinger has been accused by former undersecretary of state George Ball of exerting "obnoxious" pressure on the administration to admit the shah despite the risks to Americans in Iran. Khomeini is demanding that the shah be returned to Iran for criminal trial as the price of the hostages' freedom.

In response to other questions, the president:

Urged Americans not to vent their anger on Iranian citizens "who are in our country as guests." It would be a mistake, he said, for Americans to resort to the kind of illegal actions that have taken place in tehran.

Denied an assertion, made by Kissinger among others, that the Iranian crisis is a manifestation of declining American power in the world. Rather, the president said, the unity Americans have shown during the ordeal "is not sign of weakness but of pure strength."

Said he knows of no international forum that could try the shah for alleged crimes. But Carter encouraged anyone with claims against the deposed Iranian monarch to pursue them through legal measures that are available.

The news conference came on a day that White House press secretary Jody Powell dispatched telegrams and letters asking thousands of news organizations to encourage a public outcry for the release of the hostages. With no resolution of the stalemate in sight, the news conference itself was a part of this overall strategy to keep American opinion firmly behind the administration's stance and keep the pressure of world opinion on the Iranian authorities.

The president said that the United States has the "full support" of its allies and that virtually every country in the world, including Islamic nations, has called for the release of the hostages. Voicing what has been and remains the administration's basic approach to the crisis, Carter said he hoped that this "growing condemnation" of Iran eventually will have "a beneficial effect."