The Soviet leadership has begun a broad effort to mobilize the population for an anticipated crisis in its relations with the United States.

Well-informed sources said Communist Party members are being told in closed meetings that there is no chance of reaching an agreement at the Geneva arms talks and that the country and its economy must be prepared to meet all challenges imposed by the situation.

According to participants in the meetings, a Central Committee letter read to the party's 18 million members gives an even gloomier assessment of East-West relations than publicly stated by President Yuri Andropov.

The letter reportedly asserts that Moscow should not expect an improvement in relations with Washington soon. Its wording suggests that the Russians foresee a deterioration of relations over the next several years. This is seen as an indication that the Soviets believe President Reagan will run for a second term and win.

The letter is said to emphasize the need for greater internal discipline, vigilance and sacrifice. It focuses particular emphasis on stopping loose talk by citizens who know state secrets.

Some of these themes are beginning to filter into the Soviet media, which in recent days have criticized those "who love to prattle about state secrets in planes and buses."

There has been an unusually strong emphasis on the importance of the KGB, the Soviet secret police. The Communist Party newspaper Pravda two days ago carried laudatory remarks about the KGB that have not been heard in public for decades. It quoted a Soviet citizen as thanking "the fighters on the secret front."

"We don't know you but we have boundless faith in you. You are needed as the air and the sun. Just be clearsighted as ever and we will always help you," Pravda said.

A documentary film premiered on national television tonight portrayed the United States as a dangerous "militaristic" power bent on world domination. The 45-minute film, prepared by the Defense Ministry, contrasted scenes of U.S. nuclear explosions and various U.S. missiles with scenes of war victims, Soviet war memorials and words of Moscow's peaceful intentions.

A series of public meetings held throughout the country today also emphasized the threat to the Soviet Union posed by the scheduled deployment beginning in December of new American nuclear missiles in Europe and by Reagan's foreign policy.

At an antiwar rally in Leningrad, one of the speakers told about 400,000 persons that the international situation today is more dangerous "than at any time since World War II."

Soviet Vice President Vasily Kuznetsov, an alternate Politburo member, said at a rally in Novgorod that the U.S. leadership was "making delirious plans for world domination" and that these are "pushing mankind to the brink of disaster."

The news agency Tass tonight distributed a communique following a meeting of the Warsaw Pact Defense Council held in the city of Lvov from Oct. 26-29.

The brief communique said that the council "discussed some issues of the current activity of the joint armed forces and adopted appropriate recommendations on further perfecting their combat readiness and field training."

The Soviet media, meanwhile, continued extraordinarily sharp personal attacks on Reagan, linking the invasion of Grenada to his overall foreign policy of "international terrorism."

Two senior Central Committee officials who appeared on television tonight took obvious comfort in the fact that fighting in Grenada was continuing "at this very moment" and that Washington is finding itself politically isolated over the affair.