Vice President Mondale mounted an unexpectedly sharp attack on the Soviet Union in an address to the U.N. General Assembly's special session on disarmament yesterday, accusing the Kremlin of escalating the nuclear arms race.

In a wide-ranging review of U.S. arms control efforts, Mondale criticized Soviet activities in Africa and the Indian Ocean, accused Moscow of staging a "continuing buildup of unprecedented proportions in Europe," and attacked Soviet deployment of a moble medium-range missile known as thee SS20.

"The SS20 nuclear missile now being deployed against Western Europe is a new departure in destructive power and represents a substantial increase in the nuclear threat of the Soviet Union," he declared. "It's deployment runs totally contrary to all that this special session seeks to achieve. What can justify this escalation in nuclear arms?" he asked.

Soviets sources later expressed surprise and dismay at the unexpectedly confrontational tone of Mondale's speech, sayin g it undoubtedly would force Soviet Foreign Minister Andrei Gromyko - to respond in kind when he addresses the assembly on Friday.

Mondale's address appeared to increase the prospect that the Soviet Union will make a major propaganda issue here out of U.S. neutron weapon development.

U.S. officials had no immediate explanation for the administration's decision to take the Soviet Union so sharply to task in a forum where both superpowers were expected to come under criticism from many Third World nations for their failure to do more to halt the nuclear arms race.

The tone of Mondale's speech, however, could explain why President Carter decided not to make the American presentation in person.

Mondale, addressing delegates from 145 nations taking part in the disarmament conference, reaffirmed the Carter adminstration's committee hope that the United States and Soviet Union would reach agreement "before too long" on both a Salt II accord and a comprehensive nuclear test ban treaty.

He told the assembly that the Carter administration remains determined "to take every step possible toward greater arms control." At the same time, he added, "No nation can be asked to reduce its defenses to levels below the threats it faces."

The vice president said that while the U.S. defense budget in real terms is less than it was a decade ago, other nations have increased their spending by more than one third.

"Soviet theater nuclear forces have increased," Mondale declared. "The most significant development has been the deployment of the SS20. Each one of these missiles, which may number in the hundreds when deployment is complete, carries three nuclear warheads, each with an estimated yield of 500 kilotons."

Mondale said that beyond increasing the Soviet's military capability, "the high yield also means that damage to innocent civilians would be extensive, with effects extending 12 kilometers [more than 7 miles] from an explosion." And, he added pointedly, "the SS20 is capable of striking targets not only in Western Europe, but in Asia, Africa and the Middle East."

In reviewing other arms negotiations now in various stages of progress, Mondale also touched on talks aimed at reducing the level of military forces in the Indian Ocean.

"Increases in the Soviet naval presence there have hampered those talks," he said.

The vice president also took note of the desire of many Third World nations attending the disarmament conference to see some of the $400 billion currently being spent annually on the arms race redirected toward development projects. The United States, Mondale said, had been trying to do just that in Africa.

"My country for years sought to limit military shipments to Africa. In 1977, the United States contributed $327 million in economic assistance to African nations, compared to only $59 million in military aid," he said.

But Mondale told the disarmament conference that the redirecting funds for development is not possible unless "other nations" agree to limit their "current arming of develping nations.

"We cannot have countries pouring arms into the developing world while at the same time devoting minimal funding to developmental assistance," he said. "We cannot have nations using their military power to exploit differences between nations and to exacerbate serious conflicts."

As expected, there were few new proposals in Mondale's speech. He told the disarmament conference, however, that the United States would be willing to aid regional arms controls efforts by providing electronic systems like those used to monitor movements between Israeli and Egyptian lines in the Sinai.

"Our experience in the Middle East has demonstrated that technical assistance with monitoring systems such as aerial photography and ground detection devices can help create the confidence necessary to make this engement and stabilizing agreements work," he said.

Mondale said the United States was prepared to consider joint requests for what he called "these eyes and ears of peace" from countries that want this kind of assistance. U.S. officials said privately that this type of monitoring might prove useful in situations like the one along the Ethiopian-Somali border.

The vice president also halted the role U.N. troops have played in Lebanon, Cyprus, the Golan Heights and Sinai and called for the establishment of a U.S. peacekeeping reserve force.

He said such a reserve could be used by the secretary general whenever the Security Council decided to send a U.N. force to maintain international peace and security.