Soviet Foreign Minister Andrei Gromyko took his country's case on strategic arms negotiations to the American public today, charging in an unusual and lengthy news conference that President Reagan's proposals are one-sided and unacceptable.

Gromyko, setting the stage for the resumption of U.S.-Soviet negotiations in Geneva on June 29, accused the Reagan administration of ignoring past assessments of nuclear parity and past tradeoffs on nuclear weaponry in pursuit of unbalanced advantage.

To accept the U.S. position as outlined by Reagan, according to Gromyko, would imply "a very drastic change" in the balance of power in favor of the United States and against the Soviet Union.

In a broad indictment of U.S. policy, Gromyko also charged that the administration is determined to destroy every positive link built between the two superpowers during the era of detente.

"Whenever it sees some little bridge still in existence between the two sides, it immediately decides to throw a bomb at it and destroy that bridge, too," he declared.

With 25 years as foreign minister behind him, Gromyko is the most durable and experienced senior diplomat on the world stage, and his two-hour, 12-minute performance was a striking display of his mastery. Without referring to a single note, he was in turn somber, scornful, indignant and reassuring, now and then rolling his eyes or lifting his dark eyebrows in dismay at his adversaries.

He began by declaring that the Soviet decision to forswear first use of nuclear weapons, which he announced last week at the United Nations, "will be written with golden letters in human history."

He ended by declaring that "not a single hair will fall from any American's head as a result of any action by the Soviet Union" if relations hew to the path of "mutual respect" politically and "equal security" militarily.

In between, the seemingly ageless 72-year-old diplomat made statements on a host of subjects, saying:

* Soviet missile tests announced by Secretary of State Alexander M. Haig Jr. Saturday do not violate any agreement, formal or gentlemanly, and, despite Haig's charge to the contrary, "there was nothing whatsoever unprecedented" about them.

U.S. statements about the missile tests are "lopsided," Gromyko maintained, but he gave no details about their nature or purpose despite three questions on the subject. Gromyko said such tests will continue unless banned by mutual agreement.

* A summit meeting between Reagan and Soviet President Leonid I. Brezhnev was agreed to in principle by Haig in discussions here last week, but no date or place has been agreed to.

* A new Soviet proposal permitting on-site inspection "within certain limits" should enhance chances for agreement on prohibiting chemical warfare. U.S. charges of Soviet use of toxins in several areas of the world are "cheap propaganda," he insisted.

* The Soviet Union is "not against Israel" but is "against Israeli aggression." Gromyko charged that Israel is pursuing a policy of genocide against Palestinian Arabs. He said the Soviet Union will continue to provide "assistance to the victims of aggression," but he declined to be more explicit.

* U.S.-Latin American relations have been "somewhat spoiled" due to "the one-sided U.S. position" favoring colonialism in the Falklands conflict between Britain and Argentina, but "the Soviet Union has no self-interest in this."

The topic that drew the sharpest response was Reagan's decision last Friday to impose tight restrictions on U.S. firms or their overseas affiliates to prevent any assistance to the proposed Soviet-European natural gas pipleline.

Gromyko maintained that the step will have "no adverse effect whatsoever" on the pipeline project but added that it will leave "some kind of a sediment, some kind of an imprint" in the mind of Moscow and in the general international situation.

It was at this point that he accused the administration of seeking to destroy every bridge over the superpower gulf.

Although Gromyko has often submitted to hasty sidewalk exchanges after meetings with a secretary of state, his aides could not recall Gromyko's last full-scale news conference in the United States.

It seemed clear from his remarks, made through an interpreter at the Soviet Mission to the United Nations, that his main effort was to argue the public case for the Soviet strategic arms position.

Gromyko ridiculed the U.S. position that land-based intercontinental nuclear missiles, in which Moscow excels, are "the most destabilizing systems" and should bear the brunt of immediate reduction.

The Soviet Union, he said, could pick an area of U.S. advantage, such as submarine ballistic missiles or weapons aboard bombers, for the same description.