Soviet leader Mikhail Gorbachev offered tonight to meet President Reagan as soon as possible at a European capital to negotiate a nuclear test ban and said that he was giving the United States one last chance to halt its underground nuclear tests.

He warned that the Soviet Union would end its seven-month unilateral moratorium on nuclear weapons testing if the United States carries out another nuclear test.

In effect, Gorbachev's remarks signal the end of the Soviet moratorium because the United States has announced a test in April -- to which the Reagan administration has invited Soviet observers -- and is expected to conduct one before then.

In a 20-minute televised speech, Gorbachev stressed that a halt to testing would be "the simplest, most explicit and effective step" toward arms control.

Gorbachev repeated a Soviet offer to extend the moratorium past its March 31 deadline but only if the United States halts its tests.

"Failing that, the Soviet Union will resume testing. This must be absolutely clear," he said. "We regret it, but we will be forced to do so, since we cannot forgo our own security and that of our allies. I am saying all this so that there should be no reticence on that issue."

Gorbachev's offer to meet Reagan was viewed by diplomats here as a proposal separate from the still unresolved issue of this year's summit meeting in Washington.

The Washington summit -- agreed to when the two met for the first time last November in Geneva -- was not mentioned in tonight's speech. So far no dates for the meeting have been set, and Soviet spokesmen recently have stressed that the next summit should be aimed at producing substantive agreements.

Gorbachev's speech tonight, delivered two days before the unilateral moratorium was to expire, comes after a long Soviet campaign to stop nuclear testing.

The campaign began last August, on the anniversary of the dropping of the atomic bomb on Hiroshima, when Moscow announced that it would stop its testing program.

The original morotarium was to end last Dec. 31, but on Jan. 15, as part of a package of arms control initiatives, Gorbachev extended the unilateral step until March 31.

Two weeks ago, Gorbachev said the moratorium could continue beyond March 31 as long as the United States refrained from testing.

The United States has rebuffed the Soviet offer and has continued its nuclear testing program.

Washington argues that continued testing is necessary to ensure the reliability of U.S. weapons and that the Soviets stopped tests only after completing their own testing program.

Washington also has raised questions about verifying a test ban.

The last U.S. underground explosion, which took place in Nevada on March 22, was criticized by Soviet spokesmen here as a "blatant challenge," and a "provocation" against hopes for improved U.S.-Soviet relations.

Tonight, Gorbachev deplored what he described as the failure of the Reagan administration to follow through on "statements made in Geneva."

"Everything shows that the United States' ruling group has placed the narrow, selfish interests of the military-industrial circles above the interest of the whole of mankind and its own people," he said.

"The manner in which this is done is also quite important: pointedly, arrogantly and with disregard for the opinion of the world community," he said.

Warning that U.S. "power politics" would fail, Gorbachev said that the "Soviet political leadership is now faced with the far from simple question of how to react to such behavior from the United States."

In his answer to a message from a group of five nonaligned nations and Greece earlier this month, Gorbachev implied that the Soviet Union would resume testing if the United States tested after Monday, the moratorium deadline.

In tonight's address, Gorbachev referred to reports that the United States intends to detonate another explosion "in the next few days, in the near future."

Since August, Gorbachev has pressed for a resumption of negotiations on a total nuclear test ban. Today, he said "all variants" would be acceptable -- bilateral Soviet-American talks, tripartite talks including Britain, and multilateral talks in the framework of the Geneva disarmament conference.

"We are ready to meet President Reagan already in the nearest future in London or Rome, or in any other European capital that will agree to receive us, in order to reach agreement on this question," he said.

"We propose to meet, exchange views on this crucial problem and issue instructions to draft an appropriate agreement," he said.

Diplomats here were puzzled by the specific mention of London and Rome, although Gorbachev has been expected to go to Italy on a state visit this spring.

In his speech on Feb. 25 to the 27th Soviet Communist Party congress, Gorbachev said that his meeting with Reagan this year could be used as a forum to resolve the test ban issue or the issue of medium-range missiles in Europe.

Soviet spokesmen have insisted that Moscow is not laying down conditions for the summit. And neither the explosion in Nevada nor other U.S. actions deplored by the Soviets appear to have threatened the summit, although it may come later in the year than Washington would prefer.

Diplomats here viewed the offer of a meeting in Europe as part of the Soviet campaign on the moratorium, which -- considering the predictable U.S. response -- indicated that the Soviets are getting ready to resume testing.