The United States charged yesterday that Soviet secret police have employed "potentially harmful" chemical dust to track the movements and contacts of U.S. diplomats in Moscow and Leningrad, and demanded that the practice be stopped.

The surprise disclosure, in White House and State Department news briefings and U.S. Embassy sessions in Moscow for diplomats, their families and other Americans who might have been exposed, brought to the fore a new, bizarre and emotion-laden problem in U.S.-Soviet relations just three months before the scheduled summit meeting of President Reagan and Soviet leader Mikhail Gorbachev.

Official sources said the chemical dust -- said to be odorless, colorless and with no visible residue when properly sprayed -- was placed on steering wheels of diplomatic cars and other places where U.S. attaches would come into contact with it. The diplomats unknowingly would then leave tiny amounts of the long-lasting chemical on the hands, clothing or possessions of Soviet citizens with whom they met -- telltale traces that could be identified by the KGB, or secret police.

A few reports of use of such "tracking chemicals" -- fewer than 10 in a decade -- are said to have been in the files of the U.S. Embassy in Moscow before 1982, when their use was believed to have been stopped. No announcement of these "very sporadic" incidents was made, officials said.

Two new elements brought the problem to high level of concern, according to State Department accounts.

One was the result of a biological screening test applied in a U.S. laboratory for the first time last year to samples of the obscure compound. The Ames test, named for a professor at the University of California at Berkeley, determined that the most extensively used tracking chemical, which the State Department identified as NPPD, produced mutations in genes. Substances that cause such mutations can, but do not always, cause cancers, the department said.

Additional "extensive testing will be necessary to determine whether NPPD and other compounds used by the Soviets pose a threat to health, as well as to determine the extent of the embassy community's exposure to these chemicals," the department's announcement said.

"Any danger is far from proved," Dr. Charles Brodine of the State Department told Americans in Moscow at an embassy briefing last night, according to Washington Post correspondent Celestine Bohlen. Brodine added that initial tests "all argue that the level of risk is fairly low."

The other new element was information that Soviet use of the tracking chemicals had resumed this spring and summer on a "much more widespread basis," a department official said. Official sources said an incident in which an apparent "overdose" of the chemical left a highly visible powder, which was noticed by a U.S. aide, alerted officials to the extensiveness of the problem.

Following heavy press questioning about the timing of the announcement, the third U.S. statement this week likely to bring a harsh reaction from Moscow, a State Department official said that "only in the last several weeks" had the internal investigation produced "conclusive" results about use of the tracking chemicals.

A U.S. plan for diplomatic action, internal briefings and public announcements was drawn up Friday and presented to President Reagan for approval in a detailed paper Monday, officials said.

The department said the United States "protested the practice in the strongest terms" in diplomatic exchanges with the Soviet Union here Monday and in Moscow early Tuesday, "and demanded that it be terminated immediately."

There was no immediate comment from the Soviet Union, whose press organs continued to give heavy play to attacks on the White House announcement Tuesday of an impending U.S. antisatellite test against a target in space.

In Santa Barbara, Calif., White House spokesman Larry Speakes said it is "entirely possible" that Reagan will raise the chemical-dust issue when he meets with Gorbachev in Geneva Nov. 19 and 20.

"We will certainly discuss in various quarters the serious danger to the [U.S.-Soviet] relationship caused by the actions of the Soviet military and security services, which seem to act as if they are under no control by the political authorities," Speakes said.

It is "entirely possible" that tracking chemicals were used against U.S. diplomats without the knowledge of Soviet political leaders, Speakes said. He added, though, that "whatever the KGB has done, certainly the political leadership is responsible for the conduct of their security services."

U.S. officials said they could draw no connection between Gorbachev's accession to power in March and the apparent resumption of the chemicals' use on a larger scale sometime last spring.

Members of Congress who were reached for comment called for strong U.S. action.

The chairman and vice chairman of the Senate Select Committee on Intelligence, Sens. David F. Durenberger (R-Minn.) and Patrick J. Leahy (D-Vt.), called in separate statements for expulsion of KGB agents under diplomatic cover from the United States. Durenberger, who described himself as "still mad" 24 hours after being informed of the U.S. charges, said he had recommended that all KGB agents or suspected agents be expelled within 48 hours.