British Prime Minister Margaret Thatcher, seeking to bolster President Reagan against the potential damage of the Iran-contra hearings, said yesterday that the world needs American leadership more than ever and that Reagan "is uniquely able to give it."

"It's a time of unprecedented opportunity if we are wise and skillful enough to grasp it," Thatcher told a smiling Reagan as she left the White House after a luncheon meeting. "Now more than ever, we need American leadership, and your president is uniquely able to give it, and will give it. We must not let slip the tremendous gains of the last few years."

Thatcher, who won reelection last month to a third term as prime minister, made a 12-hour visit here that had been billed by British commentators as an attempt to help Reagan break free from the Iran-contra revelations and get his administration into high gear on such foreign policy initiatives as arms control, Middle East peace and world economic prosperity.

She has been regarded as Reagan's closest friend among foreign leaders, and the two agree on many broad issues of social and economic policy. Interviewed on NBC's "Today" program yesterday about U.S.-Soviet negotiations to reduce medium-range, nuclear missiles in Europe, Thatcher said:

"I believe he's a great leader. Even during this difficult period, he has not let go his leadership role in any way . . . . I don't think {the Soviets} will find a politically weakened president. I have absolute trust in him."

Neither Thatcher nor Reagan mentioned the Iran-contra hearings directly. After Reagan saw Thatcher off at the White House, reporters shouted questions about the hearings to the president. He replied, "I will have a statement to make when the investigation is over," and walked back into the White House.

Reagan earlier told reporters that he and Thatcher share "a high degree of agreement" on such issues as arms control, direct Arab-Israeli peace talks in the Middle East and ending the Iran-Iraq war in the Persian Gulf.

The U.S. and British governments are backing efforts in the U.N. Security Council for a resolution demanding a cease-fire between Iran and Iraq. The council is expected to adopt the resolution next week. Reagan said he and Thatcher want U.N. Secretary General Javier Perez de Cuellar to underscore its intent with a personal mission to the area.

"If either or both of the warring parties should refuse the U.N. call for a cessation of the fighting, an arms embargo should be brought to bear on those who reject this chance to end this bloody and senseless conflict," Reagan said.

On the Middle East, Thatcher said she and Reagan had explored how an international conference might help bring about direct talks between Israel and Jordan.

In regard to arms control, Thatcher said she and Reagan had agreed on the need for reductions in nuclear weapons without destroying the West's maintenance of effective deterrence against the Soviets. In past meetings, Reagan has trimmed his arms control initiatives to accommodate various specific concerns of Thatcher's, such as her insistence that any agreements have very strict provisions for verification. She, in turn, has backed U.S. proposals in this area.

However, one point on which there seems to be a detectable degree of difference between the two leaders involves the background against which arms talks and other efforts to improve superpower relations are taking place. In recent days, U.S. officials have complained that the Soviets are stalling or engaging in a "pause" that threatens hopes for a quick agreement on a summit meeting this fall between Reagan and Soviet leader Mikhail Gorbachev.

But Thatcher is known to feel that Gorbachev is not pulling away from a desire for accommodations with the West but instead is being forced to reduce the pace in order to address internal problems. In her White House remarks, she said: "Great changes are taking place in the world, including historic changes in the Soviet Union."