President Reagan was urged by allied leaders today to respond to the recent Soviet proposal for a 50 percent cut in nuclear weapons with a new proposal of his own as he prepares to meet Soviet leader Mikhail Gorbachev in Geneva next month.

A few hours after Reagan delivered an address to the United Nations that stressed Soviet involvement in regional conflicts in the Third World, he was reminded by the western leaders that arms negotiations remain their foremost concern as the summit approaches.

British Prime Minister Margaret Thatcher suggested to Reagan during an hour-long minisummit with the other leaders that the United States should "repackage" its current arms reduction proposals in response to the latest Soviet initiative, according to a senior U.S. official who was present.

Thatcher's remark reflected a wider concern among the allies that the West not allow the Soviet proposal for a 50 percent reduction in nuclear weapons to go unanswered in the intensified superpower jockeying for world public opinion before the November summit.

Thatcher's remarks in a British television interview also reinforced the view that allied priorities lay in arms control, an area in which she said it is "vital" that a new impetus be given to break the "deadlock in Geneva." While expressing understanding for the regional peace initiatives Reagan proposed today, she said, "I think it requires a great deal of thinking about before we dash into comments about it."

At the U.S. Mission to the United Nations, Reagan met with Thatcher, West German Chancellor Helmut Kohl, Japanese Prime Minister Yasuhiro Nakasone, Canadian Prime Minister Brian Mulroney and Italy's caretaker prime minister, Bettino Craxi.

The meeting was designed to be a display of western unity in advance of the Nov. 19-20 summit in Geneva, and most of the participants spoke afterward in unusually harmonious terms. An agreed statement by the leaders stressed that they gave Reagan their "full support" for his Geneva meeting.

But they also attempted to insure, as Thatcher put it afterward, that "arms control is not being downgraded as a summit priority."

"Well, it isn't at the bottom of the agenda," said Secretary of State George P. Shultz. He said that the leaders expressed hope for progress on "the full range" of U.S.-Soviet issues in Geneva "and that it would give a strong impetus to current negotiations and, particularly, of course, the arms control discussions in Geneva."

Other participants echoed Shultz and went further to say that the United States needs to make a new proposal in Geneva. However, none of the leaders was specific about the shape of such a new or "repackaged" proposal. Rather, they appeared to be commenting on their view that the Soviets not have the last word before Geneva.

A White House official who listened to the discussion said it was not clear whether Reagan would take their advice to make a new proposal. But the official said Reagan may devote a speech to arms control before leaving to meet Gorbachev.

Noting the Soviet proposal for a 50 percent cut in nuclear weapons, and this week's offer in Geneva of a freeze on deployment of intermediate-range missiles in Europe and Asia, Craxi told reporters: "There are Soviet proposals which contain some positive elements. Now it is necessary to have counterproposals to reduce" nuclear weapons stockpiles.

Craxi's attendance at today's minisummit followed U.S. mending of relations strained over Italy's actions in the aftermath of the hijacking of an Italian cruise ship on Oct. 1. Craxi's five-party coalition fell when Defense Minister Giovanni Spadolini withdrew his party, citing disagreements with Craxi's handling of the crisis. Today, a senior administration official said Reagan and Craxi had met as friends.

On the subject of a counterproposal, the White House official said Thatcher had taken the initiative. But it was clearly shared by other leaders attending.

An aide to Kohl said the West German chancellor, who meets with Reagan Friday, believes Reagan should make an arms cut "counterproposal" before Geneva and expects an "active" U.S. role in the area of arms talks before the summit.

The aide said as an alternative, Reagan might announce such a counterproposal at the summit. Kohl also wants Reagan to address other arms reduction topics, including the stalled chemical weapons talks in Geneva.

Reagan's national security adviser, Robert C. McFarlane, asked in an interview with NBC this morning about the prospects for an arms accord in Geneva, said "the complexities of it will require some time to be negotiated out." But, he added, "It is conceivable that there can be a meeting of the minds on principles and objectives . . . . "

Kohl sent Reagan two letters on the summit with suggestions on arms control, the West German official said. One urges Reagan to consider negotiating a "code of conduct" for superpower relations. Such a code was adopted at the 1972 Moscow summit between President Nixon and Soviet leader Leonid Brezhnev, but Reagan aides have said they consider it unlikely in the Reagan-Gorbachev meeting.

Kohl also reportedly suggested that Reagan strive to make superpower summits a regular event.

Nakasone also stressed arms control in remarks to reporters after the meeting. He said the upcoming Soviet Communist Party congress in February is one factor providing inpetus to both superpowers to make the summit fruitful.