BRUSSELS, MARCH 2 -- NATO leaders, seeking to test Soviet willingness to give up the Warsaw Pact's longstanding advantages in conventional forces in Europe, called today for the next stage of arms control to establish a military balance between the rival blocs by cutting arsenals of military equipment such as tanks and artillery.

With one nuclear arms treaty signed and negotiations progressing on another, President Reagan told the North Atlantic Treaty Organization summit here that the talks on nonnuclear forces are "our next priority as an alliance."

In an important shift from NATO's stance in previous negotiations on conventional forces, a six-page statement said the talks should focus on reducing military equipment rather than troops, as in earlier talks now winding down after 15 years with little progress.

The purpose of the change is to seek to eliminate the Warsaw Pact's capability to stage a surprise attack on Western Europe. The Soviet Bloc should accept "the elimination from Europe of tens of thousands of weapons" of the type that could be used for such an offensive, the statement said.

The NATO leaders also said advances could not be achieved in the conventional arms negotiations without "substantial progress" on European political and economic cooperation and on human rights, as addressed in the 1975 Helsinki accords.

The 16-nation summit was called largely to demonstrate NATO unity at a time of change in the East-West relationship, and leaders generally sought to minimize their differences over such issues as future modernization for European-based nuclear arms.

An exception was British Prime Minister Margaret Thatcher, who strongly reaffirmed her view that "we must keep our arms up to date." Her comments clearly were directed at West German Chancellor Helmut Kohl, who opposes making specific commitments now on modernizing short-range nuclear arms.

But British and U.S. officials said that Thatcher had agreed for the time being not to force the short-range modernization issue. Instead, Britain and the United States will seek to commit NATO in coming months to strengthen the alliance's force of longer-range, European-based weapons, they said.

"There are no great fundamental differences," Reagan told reporters after the afternoon session. "I have never seen such harmony and togetherness as we have."

The statement on conventional forces outlined NATO's objectives for talks that are just getting started in Vienna. Negotiators there, representing 35 countries, are seeking to establish ground rules for what the talks will cover.

NATO had agreed previously, most recently in December 1986, on some guiding principles for the negotiations. But the statement here, by providing more details, was designed to emphasize NATO's increasing desire for negotiations on nonnuclear forces.

"The conventional imbalance in Europe remains at the core of Europe's security concerns," the statement said. The dominant position of Soviet troops casts "a shadow over the whole of Europe," it said.

Interest in talks on conventional weapons has been spurred by the signing in December of the treaty covering Intermediate-range Nuclear Forces (INF) and by the prospect that the United States and the Soviet Union may soon reach a treaty providing for 50 percent cuts in long-range nuclear missiles.

Many NATO governments believe that NATO should not negotiate away any more of its nuclear weapons as long as the Warsaw Pact enjoys a superiority in conventional forces.

"The conventional arms talks will be the most important single test of whether Gorbachev is serious about making a fundamental change in East-West relations," a senior West German official said.

In addition, progress in the conventional-arms talks could be more likely, given Soviet willingness under Gorbachev to make concessions that they previously had resisted.

In the INF treaty, for example, the Soviets agreed to make larger reductions in their military capability than the United States. The NATO statement here said that "highly asymmetrical reductions" in conventional forces would be necessary to establish a balance in Europe.

NATO estimates that the Warsaw Pact has at least a 3-to-1 advantage in tanks and artillery.

The Soviets also have become much more willing to accept on-site inspections to prevent cheating on arms accords.

Finally, the emphasis on NATO's interest in conventional-arms talks also could help assuage some concerns in the U.S. Senate as it debates ratification of the INF treaty. Some senators, including Sam Nunn (D-Ga.), have expressed concern that removing all of NATO's intermediate-range missiles, as the treaty provides, would leave the western alliance dangerously exposed to the Soviets' conventional superiority.

Despite NATO's enthusiasm for conventional-forces talks, the negotiations face some formidable difficulties.

They cover more territory in Europe, and involve more countries, than the earlier negotiations. Those were called the Mutual and Balanced Force Reduction Talks.

In addition, NATO offered no concessions on two major sticking points. The alliance did not offer to discuss tactical aircraft, where the Soviets claim that the Americans have superiority in some areas. NATO also did not agree to leave open the possibility that the conventional-arms talks eventually would deal with European-based, short-range nuclear weapons, as the Soviets have insisted.

Washington Post correspondent Karen DeYoung in London contributed to this report.