Only minutes after smiling and bidding farewell to President Reagan, Mikhail Gorbachev went before live television cameras today to describe exchanges with the American leader that were sometimes "very lively. At one point it actually became very, very, lively indeed."

The 54-year-old Soviet leader, displaying again the type of presence before the camera that has long been equated with the man in the Oval Office, gave every indication that the five hours the two spent together here during the past two days were filled with earnest and occasionally tough conversation.

For an hour and 40 minutes he alternately spoke of blunt warnings and fervent hopes for the future. Comparing his summit sessions with Reagan to the start of a rescue mission, Gorbachev said that he and his Kremlin colleagues had approached the Geneva talks with the clear recognition that serious problems had developed in U.S.-Soviet relations.

"When you find the path is blocked if you're a geologist or a miner, then a special brigade is called up to decide how can we save the situation, how can we save the people who are stuck behind this landslide . . . ," Gorbachev said.

"And we, too, we must decide how to save our relationship from the stress that has built up, how we must go away from confrontation into a more healthy way of solving problems. This work we must do through our united efforts.

"We are prepared for this sort of united effort," the Kremlin chief went on, adding that "I told the president that it would be a great shame and a mistake if we don't make use, you and me, of the opportunity that has appeared for us to change the direction of the situation we find ourselves in."

In his presentation before about 300 journalists, the Soviet leader displayed the kind of aggressive confidence that Secretary of State George P. Shultz said he had noted during their recent talks in Moscow, and he showed no hesitation about placing himself on equal footing with Reagan.

" . . . We two leaders are responsible for the future of the world, for the continued existence of life on our planet," he said at one point.

"At such a watershed you need truth, like you need air to breathe," he continued. "And there is no hiding from the truth when you are meeting face to face."

It was the tougher image that Gorbachev clearly wanted to project, as it was this approach that dominated his one-hour prepared presentation, while the voice of greater reason emerged in his answers to reporters' questions.

Although the meeting brought about no change in weapons stockpiles on either side, the Soviet leader said, "I would say that the world has become a more secure place" as a result of the opening of a personal dialogue for the first time between the two leaders.

Gorbachev confirmed that his nine hours of talks with Reagan had done nothing to budge the deadlock on "Star Wars" -- the American high-technology missile defense initiative, which the Soviets seem determined to block and which was the key issue for them at the summit. He made clear that in the Soviet view, Star Wars still dominates arms negotiations.

"We are prepared to engage in radical reductions [of offensive arms] provided the door to the arms race in space is closed and tightly shut," he said.

But he also gave an upbeat assessment of the summit, which he said had accomplished "serious work" through "open and frank" exchanges. In sum, he said, the meeting "lays the groundwork to mutual understanding and a dialogue." And that, he added, "is conducive to enhanced security."

Gorbachev said he hoped that the summit effected a changed impression of the Soviet Union and its leaders by the Reagan administration, but he declined to say whether his own views of the United States changed as a result of the summit meetings. Earlier this month, Shultz and national security adviser Robert C. McFarlane reportedly were surprised during a visit to Moscow by the strength of Gorbachev's conviction that there was deep-rooted anti-Soviet sentiment in the U.S. administration.

In his speech preceding the press conference, Gorbachev went into a detailed, animated account of his discussion with Reagan on SDI. He said Reagan told him that SDI was a defensive rather than offensive weapons system, "but that is not the way it is at all. We've already said we won't strike first. So I asked," Gorbachev continued, "Why are you taking the arms race into further spheres? You don't believe us. Why should we believe you when you say these weapons are defensive?"

"We hope it's not the U.S.'s last word," Gorbachev said twice, possibly looking to next year's summit or what both leaders said would be an "accelerated" pace of arms negotiations. If the Reagan administration continues with its SDI plans, Gorbachev warned, Moscow will respond, and the response "will be effective."

Gorbachev's remarks were broadcast live to the Soviet Union. Western analysts here said the Soviet leader's hard-knuckled remarks on arms control were intended largely "for domestic consumption."

But Georgi Arbatov, a chief Soviet specialist on America, said in an interview, "It cannot work the way the Americans think," meaning there could be no offensive cuts if the U.S. defense shield goes ahead.

According to Gorbachev, "something of a fight" broke out when the U.S. side pressed its charges of Soviet interference in regional disputes. So it was decided from the outset not to engage in such "banalities," he said.

Reagan, in a speech last month, said that Soviet or Soviet-backed military involvement in five Third World conflicts would be his main priority in the summit.

In his account, Gorbachev said the two sides did "touch upon regions" but "we spent most of our time looking at the principles behind these various regional issues."

He called for Soviet-American business contacts and listed the various bilateral agreements reached, but he did not mention discussions on human rights, which were also on the summit agenda.