Sen. Edward M. Kennedy (D-Mass.), appearing on Soviet television today, called the Kremlin's recent arms control proposal "constructive" and reiterated his support for a comprehensive ban on nuclear testing.

Kennedy also voiced reservations about the Reagan administration's Strategic Defense Initiative, which the Kremlin opposes, but said he was certain that President Reagan was willing to negotiate "a real reduction in nuclear weapons."

The 40-minute broadcast was the first extensive Soviet television appearance by an American official in recent memory. It consisted of a 10-minute prepared statement and a 30-minute interview of Kennedy by Soviet journalist Vadim Zorin.

Kennedy referred in his opening statement to Afghanistan, human rights issues and other areas where the United States and Soviet Union "find ourselves at odds."

The broadcast was taped Friday during Kennedy's three-day visit here, a U.S. official said. Parts of it were excerpted and broadcast on the Soviet evening news that evening.

American officials' appearances in the Soviet news media are rare, although Soviet television carried a short New Year's greeting by President Reagan, and a Reagan interview was published by the government newspaper Izvestia last November.

Kennedy shunned the western press during his three-day Moscow visit and gave no press conference here. After returning to Washington, he held a press conference Saturday in which he said that Soviet authorities had agreed to resolve some additional human rights cases and that he had spoken forcefully to Soviet officials about their treatment of two prominent Soviet dissidents -- Anatoly Scharansky and Andrei Sakharov.

In Moscow, Kennedy granted an interview to an official Soviet newspaper, Sovetskaya Kultura, in addition to the television broadcast.

Zorin used the television appearance to ask about Kennedy's positions on various arms control initiatives, such as a nuclear test ban and SDI, the Reagan administration's research program for a space-based missile defense system. Kennedy repeated his widely known positions, which differ from those of the Reagan administration.

In his statement, Kennedy said the Soviet Union and the United States should "spend less time preparing for a nuclear war and more preventing one."

"Let us compete," Kennedy said, "and where we can, cooperate, in pursuing the works of peace."

Kennedy said his Thursday meeting with Soviet leader Mikhail Gorbachev was "cordial and friendly." He called Gorbachev "a cheerful, energetic and knowledgeable leader who is clearly aware of the importance of the endeavors to reach agreement on . . . nuclear arms."

Following the November talks between Gorbachev and Reagan, Kennedy said, "for the first time in some time . . . we have seen the promise of a more peaceful world."

Kennedy met with Reagan before going to the Soviet Union and with Gorbachev at the Kremlin last Thursday.

"I believe that General Secretary Gorbachev is ready to negotiate a real reduction in nuclear weapons," he said. "I know that President Reagan is willing to enter into such an agreement."

Both leaders have spoken of "the same dream of a world without nuclear war," he said.

"Making that dream a reality," he added, "requires that both sides search for a compromise to cut through the negotiator's Gordian knot."

Kennedy said that in his talks with Soviet officials he "detected a real seriousness." In addition to Gorbachev, he met with Foreign Minister Eduard Shevardnadze and others.

During the broadcast, he and Zorin joked as they sat across from one another with a large vase of flowers between them.

In a reference to World War II, he said that his brother, the late president John F. Kennedy, had been "mindful of the sacrifices of the Soviet people during that conflict."

"The Soviet Union and the United States have endured tension, confrontation and deadlock at the bargaining table, but we have never stepped over the brink into humanity's final suicidal war."