President Reagan is expected to pledge to the freed American hostages and warn the world today that "never again" will the United States allow its diplomats to be taken captive with impunity.

White House sources said that Reagan would make this pledge in a welcoming speech to the released hostages, whom the president considers former "prisoners of war."

Reagan was described as being visibly moved yesterday when told by two of his Cabinet members that about a dozen of the Americans were suffering from "severe problems," mostly mental in nature, because of mistreatment during their captivity in Iran.

White House press secretary James S. Brady said that Reagan was "a little watery-eyed" when he was given details of the treatment and condition of the former captives in a morning briefing by Secretary of State Alexander M. Haig Jr. and Secretary of Defense Caspar W. Weinberger.

Reagan's feelings toward the released hostages came through yesterday when he signed a proclamation passed by both houses of Congress asking Americans to give thanks for the return of the Americans. The president said the hostages had shown by their example "that the spirit of our country will never be broken."

The hostages will be welcomed by Reagan in a ceremony on the White House South Lawn that is expected to be attended by 6,000 people, most of them the families of the hostages, their colleagues from the Defense and State departments and members of Congress.

Reagan's remarks, which the president worked on last night, are expected to be both brief and pointed. While Reagan is not expected to spell out specific changes of policy, he is likely, as one White House aide put it, to issue "both a welcome and a warning to others" against the kidnaping of Americans abroad.

"He remains outraged by what happened," said an aide. "He wants to make certain that people know that there would be a different reaction and not procrastination if this ever happened again."

In signing the congressional resolution honoring the former hostages, Reagan also paid tribute to the "devotion and bravery" of the eight military men who lost their lives in attempting to rescue them last April. The families of these men have been invited to attend the ceremonies for the returning hostages at the White House.

Speaking of the hostages, Reagan said: "This resolution pays tribute to the strength of America. It recognizes the principle of public service which 53 men and women fulfilled in the highest tradition of their calling."

Exactly what Reagan proposes to do if any other nation emulates Iran's conduct in violating diplomatic immunity isn't clear. Reagan's frequent use of the phrase "prisoners of war" is considered by some to be a clue that he would regard any such action as an act of war, but Brady said yesterday that isn't necessarily the case.

The entire question of what to do in response to future acts of terrorism is under review, said Brady. But Reagan is likely to give only the broad outlines of such a policy today.

Also under review is the agreement President Carter signed with Iran to win release of the hostages. Reagan is not expected to repudiate this agreement, but White House aides are carefully examining the fine print.

An announcement on the Reagan administration's view of the agreement could come within days, perhaps as early as the president's first news conference, now planned for Thursday or Friday.

Not all of the attention at the White House yesterday concerned the former hostages. Reagan continued his series of meetings with congressional leaders, and foreign policy shared the limelight with economic issues.

One Republican senator who met with Reagan, William Armstrong of Colorado, said he urged the president to take advantage of the favorable climate toward his programs in Congress to submit "a single, well-defined economic package" that includes tax cuts, spending reductions, regulatory reforms and perhaps some changes in budget-making policy.

The latest tentative date for submission of this package is Feb. 17, and it won't be a day too soon for congressional Republicans who want Reagan to take advantage of his honeymoon.

"The caution I gave him is that anything that comes this spring or summer is too late," Armstrong said. "Now is the moment."

Three other conservative Republican senators met with Reagan yesterday -- James McClure of Idaho, Barry Goldwater of Arizona and Jesse Helms of North Carolina.

Helms opposed the appointment of Weinberger and particularly of Weinberger's deputy, Frank Carlucci, who was approved for confirmation yesterday by the Senate Armed Services Committee. Helms' opposition was based on ideological grounds and their purported inexperience on defense issues, but he emerged from the White House yesterday in a happy mood because, he said, conservative Fred Ikle, a principal Reagan defense adviser during the campaign, will get the No. 3 post at the Defense Department.

Helms also said he didn't think it important that the United States live up to the agreement that won release of the hostages.

"I don't think the national honor of the United States hangs on the thin thread of living up to anything involving a bunch of bums," he said. "As far as I'm concerned, they could take a long walk on a short pier."