MOSCOW, DEC. 14 -- Soviet leader Mikhail Gorbachev, speaking to the nation on his three days of talks in Washington with President Reagan, said today that important differences over U.S. plans for a space-based missile defense system still divide their two governments and he warned that the conflict could derail the "nascent" process of nuclear disarmament.

In a nationally televised report at the start of the city's evening news program, Gorbachev called on both sides to maintain the "new atmosphere" in U.S.-Soviet relations resulting from the summit meeting and urged the U.S. Senate to ratify the treaty to eliminate medium- and shorter-range missiles that was signed in Washington last week.

Reading somberly from a prepared text, Gorbachev hinted that some members of the Kremlin leadership were not fully convinced by the compromising stance Reagan took during talks. He said that the delegation carefully analyzed whether there were changes in the Reagan administration's approach to the Soviet Union during the talks.

Saying the issue was "not easy to comprehend," Gorbachev continued, "I should tell you . . . it is so far early to speak about a drastic turn in our relations."

"Nevertheless," he added, "I want to say that the dialogue with the president and other political figures of the United States was different than before. It was more constructive."

Gorbachev used the 20-minute speech to express caution about jumping to conclusions that Reagan's plans for a space-based Strategic Defense Initiative, or antimissile shield, can now proceed full steam ahead.

"Certain persons even try to assert that the talks in Washington have removed differences on such a problem as SDI and under that pretext make calls for speeding up work on that program," the Soviet leader said.

"I must say outright that these are dangerous tendencies and that they should not be underestimated," he said, adding that they can "undermine the nascent turn" in disarmament.

Gorbachev's forceful warning against SDI, coming two days after Reagan defended the program in a radio address, suggested that the issue of space defense could flare up again, although both leaders managed to avoid a clash over it during their three days of talks last week.

Gorbachev admitted in his Washington press conference last week that no progress had been made during the talks in what he describes as his battle to avoid an arms race in space.

In today's speech, Gorbachev stressed that Soviet approval of a new treaty under negotiation between the two sides to cut strategic weapons by 50 percent on both sides was conditioned on a U.S. agreement to adhere to the 1972 Anti-Ballistic Missile treaty, which restricts testing on antiballistic weapons systems.

The talks in Washington ended on a more positive note than the encounter between Gorbachev and Reagan in Reykjavik, Iceland, in October 1986, and produced more substantive results than their first meeting in November 1985 in Geneva.

In contrast to the free-wheeling, open appearances he made in Washington last week, Gorbachev was measured in his remarks tonight, often pausing to look down at his text.

"We have reaffirmed our readiness for a 50 percent cut in the strategic offensive weapons on condition the ABM treaty be preserved in the form it was adopted in 1972," he said.

Gorbachev urged support for the treaty on intermediate nuclear forces, which must be approved by the U.S. Senate and the Supreme Soviet before the missiles covered by it are to be dismantled. "We need to give legal form to the treaty, to ratify it," he said, adding later that he knew there was a "struggle" over it in the United States.

Emphasizing that "the new atmosphere in Soviet-American relations needs to be maintained," Gorbachev appeared to be campaigning for support for his own program of disarmament.

"Merely three days have passed since our return," Gorbachev said, "but definite circles in the United States and in other western countries are already rallying to prevent the changes for the better."

"Voices calling on the leadership of the United States not to go too far, to halt the process of disarmament, sound ever louder," he added.

But Gorbachev said he hopes "sound forces in all countries will redouble their efforts to save the first sprout of nuclear disarmament that pushed through concrete walls of prejudice and stereotypes of hostility."