The Soviet Union tonight acknowledged for the first time that a South Korean jumbo jetliner that disappeared last week with 269 people on board was downed by a Soviet fighter plane after intruding into Soviet airspace.

The belated acknowledgement by Moscow came five days after the incident that has soured East-West relations and provoked a torrent of condemnations in the United States and other western countries. But a Soviet government statement reiterated earlier claims that the downed plane had been engaged in a "spying mission" for the United States and insisted that "entire responsibility for this tragedy" rested with American leaders.

Moscow's official position, as reflected in tonight's statement, is that the whole affair was deliberately engineered by "American special services" in the knowledge that innocent passengers were being exposed to mortal danger. The plan, according to the Kremlin, was to use a civilian plane to conduct a "major intelligence operation" over Soviet territory and--if that went wrong--"to turn all this into a major political provocation against the Soviet Union."

In Washington, the Reagan administration denounced the Soviet government statement as "lies and half truths," and called for an "unequivocal apology" and reparations.

At the same time, the White House backed off from earlier statements that the United States had "irrefutable evidence" that the Soviets knew the plane was a civilian airliner but said that if they didn't know, "they should have." Details on Page A13.

Tonight's communique, which was read out as the lead item on state television news, was regarded by western diplomats here as the most authoritative statement yet on the downing of the plane from the Kremlin leadership. It was the first time that the Soviet government--as opposed to the official news agency Tass--has commented formally on the affair and can be viewed as a direct response to President Reagan's television appearance last night in which he accused Moscow of staging "a massacre."

The communique called Reagan "an ignoramus."

In a report from Washington, Tass tonight called Reagan's speech "slanderous," "mendacious" and "hypocritical," and said it was "imbued with pathological anticommunism."

The increasingly harsh language used by the Soviets suggests that the Kremlin has decided on a policy of matching President Reagan's accusations in kind and has no intention of apologizing or punishing the officials who ordered the downing of the plane.

The government statement did not directly mention Korean Air Lines Flight 007, talking instead about "an intruder plane." But earlier statements, including an interview with the head of Soviet antiaircraft defense, have explicitly used this term in connection with the South Korean Boeing 747 that disappeared Thursday.

Describing the sequence of events that led to the plane's disappearance, the Soviet authorities tonight said it had failed to respond to a series of warnings from Soviet fighter aircraft and had even tried to evade pursuit.

The statement went on: "The interceptor-fighter plane of the Soviet antiaircraft defenses fulfilled the order of the command post to stop the flight. Such actions are fully in keeping with the law on the state border of the U.S.S.R., which has been published."

The statement did not specify exactly how the order was "fulfilled." But it is clear that the Soviet fighter fired at the target.

It was the first time that ordinary Soviet citizens have been told officially that the Soviet fighter trailing the jet had done anything more than fire warning shots.

The U.S. Embassy tonight described the Soviet statement as "much too little and much too late."

"While the Soviets have finally been compelled by the weight of evidence to admit that they shot down the Korean airliner, virtually every other element in their statement is obviously designed to evade full responsibility for the atrocity they have committed," the U.S. statement said.

The Soviet government announcement implied that the decision to shoot the plane down had been made by the "antiaircraft forces command of the area" after concluding that the jetliner was "a reconnaissance aircraft." There was no mention in the 800-word account that the matter was ever referred to the civilian authorities in Moscow.

Justifying the decision, the Soviet government said that its pilots had been unable to tell that a civilian aircraft was involved since the Korean plane was "flying at the height of night," "without navigation lights," in "conditions of bad visibility," and "was not answering the signals" of the Soviet planes.

"We will continue to act in keeping with our legislation, which is fully in accord with international regulations. This wholly applies to the question of ensuring the security of our borders," the Soviet statement added.

The statement made clear that the Soviet air defense authorities did not confuse the South Korean plane with an American RC135 reconnaissance plane that the United States has acknowledged was in the area at the time. It said that "several Soviet interceptor planes" were sent up, one of which monitored the actions of the American RC135 while another followed the "intruder plane" that had overflown a major missile base in Kamchatka.

The Soviet government said that efforts to make contact with the South Korean plane had included broadcasting with a generally known call signal on an international emergency frequency of 121.5 megacycles. It denied a claim by President Reagan that to guard against defections by pilots, the Soviets do not equip their fighter planes with that frequency.

Accusing President Reagan of whipping up "anti-Soviet hysteria" to distract attention from the arms race and the threat of nuclear war, the Soviet statement said: "The people on the plane that was used by American special services for their dirty aims fell victim to a fresh crime."

In a reaction to Reagan's television appearance last night, Tass said the U.S. president was shedding "crocodile tears" to disguise U.S. atrocities in Vietnam, Lebanon and other places around the world. It said that if Reagan really wanted to promote the safety of civil aviation, the United States should stop sending its warplanes "to peek in foreign lands."

The Soviet Union also has formally protested to the U.S. Embassy over what it called "a criminal bandit attack" two days ago by "a mob of rampant hooligans" against the residence of the Soviet ambassador to the United Nations. Tass said that American policemen did nothing to protect the building and that none of the intruders was arrested.