Soviet President Leonid Brezhnev today accused President Carter and the United States of spreading "mountains of lies" about the Soviet intervention in Afghanistan and denounced the United States as an absolutely unreliable partner" in world affairs.

In his first public statement since the Dec. 27 Soviet-backed coup in Afghanistan, Brezhnev reinforced the Kremlin's contentin that harsh U.S. reaction, not the Soviet intervention itself, was responsible for plunging East-West relations to a dangerously low point.

Although Brezhnev reiterated many points already made by the Soviet press and officials, the personal condemnation of the United States and defense of the Afghan policy by the Soviet president and Communist Party leader dispelled any doubts about the Kremlin's unity on the issue.

Brezhev's statement came as part of a series of lengthy "answers to questions of a Pravda correspondent" to appear in Sunday's editions of the official Communist Party daily. It was also published by Tass news agency today, broadcast on Soviet radio and reported twice on television, indicating the importance attached to it.

While echoing previous Soviet statements that Carter's retaliation will harm the United States more than the Soviet Union, Brezhnew's statement nevertheless implies that the Kremlin has been stung by the U.S. moves.

In an unusual specific departure from the foreign policy thrust of his remarks, Brezhnev declared, "in particular, I can assure that the plans of providing Soviet people with bread and bakery products will not be lessened by a single kilogram."

While most Sovietrs have only bare knowledge that Carter, as one of the U.S. retaliatory moves, embargoed 17 million tons of feed grain for Soviet meat herds, some unofficial sources have said peasants and workers are talking of hoarding in anticipation of shortages, Brezhnev's virtual pledge seems intended to blunt that impulse which, of itself, could create shortages.

Brezhnev said that in sending troops to Afghanistan, the Soviet Union was merely responding to repeated pleas from the Afghan regime for help in repelling forein subversion of the April 1978 Marxist revoltuon.

He called any other description of it "mountains of lies," and said, "Thousands and tens of thousands of insurgents, armed and trained abroad -- whole armed units -- were sent into Afghanistan."

Developing the Kremlin's theme that the United States, under Carter, cannot be trusted as a partner in detente, Brezhnev said American policy decisions now are "prompted by some whim, caprice or emotional outbursts, capable at any moment of violating international obligations and canceling treaties and agreements." He added, "There is hardly need to expain what a dangerous, destabilizing impact that has on the entire international situation."

Whatever good will may have developed at last June's Vienna summit, when the two leaders embraced and kissed after signing the SALT II treaty, clearly has been blown away by the invasion, in the view of observers here.

Carter publicly questioned Brezhnev's veracity after a telephone "hotline" exchange Dec. 28, and today Brezhnev complained bitterly that Carter has "arbitrarily and unilaternally violated" so many Moscow-Washington accords that "it is difficult even to enumerate [them]." He said Carter's actions were a "poor attempt to use Afghanistan to block efforts to lessen military danger, strengthen peace, restrict the arms race." This line also seemed intended to explorit any nervousness among U.S. allies about Washington's long-range intentions.

Brezhnev contrasted American militarism and dismal bilateral relations to what he said is an optimistic state of relations between Moscow and West European governments, despite U.S. attempts to "spoil" these relations. The Kremlin has spoken with increasing boldness to the West Europeans since NATO decided last month to improve its nuclear missile forces. The Kremlin is pursuing two lines here: it has refused to consider theater disarmament talks until the NATO decision is reversed, but also emphasizes political detente and has warned the Europeans against doing "the bidding of politicans from across the ocean," as Brezhnev put it today.

The Kremlin has never said whether it intends any specific retaliation of its own in response to Carter's reactons and most Western sources here believe it will be nothing except mount a storm of propaganda. But Brezhnev made what could be interpreted as a veiled and weak threat today when he said Carter's reprisals, "like a boomerang, will hit back at their initiator, if not today, then tomorrow."

Brezhnew said nothing of reports of a potential U.S. boycott of the Summer Olympics. Few interational events have meant so much to Moscow as having the games here next July. Vast effects have been mounted to showcase Soviet life as the model for the world, and moscow has hailed them as proof of world acceptance of the quality of the first communist state with all other nations.

He also continued Kremlin policy of making no answer to other governments and foreign Communist parties that have condemned the Soviet action, and he called U.N. debate on it illegal, external meddling in Afghan affairs.

This underscores Kremlin attemps to extricate itself from having to justify the use of Soviet arms to back a regime that is at war with Moslem tribesmen, who have many sympathizers in the Third World. Brezhnev labeled the insurgents imperialist fighters, in keeping with a propaganda shift that no longer recognizes them as Moslem faithful disaffected by the repressive Hafizullah Amin, Barak's predecessor.

Brezhnev said the White House worked with China to topple the Marxists and make Afghanistan "an imperialist military bridgehead on our country's southern border . . . a seat of serious danger to the security of the Soviet state." In another declaration that points up the Kremlin's propaganda difficulties, however, he said, "The national interests or security of the U.S. and other states are not affected in any way by the events in Afghanistan. All attempts to portray matters otherwise are sheer nonsense."

He accused the United States and Cina of planning new arms for the insurgents, and called "absolutely false" any suggestion that the Soviet Union "has some expandionist plans [against] Pakistan, Iran or other countries of that area." Meanwhile, he said, the United States, "attracted by the smell of oil," has brought atomic-armed ships to Iranian shores to coerce Tehran. He did not mention the American hostages there.

The "cold-war language" of Carter's reprisals has stalled the SALT II treaty, which "would have opened the way to big disarmament steps," Brezhnev said, and now the international situation "unfortunately has noticeably deteriorated."

Recalling earlier capitalist moves against the new Soviet state, Brezhnev appealed to his compatriots patriotism while warning Washington, "If all these sallies against our policy are intended to check our mettle, history is totally ignored."