The Soviet Union today broadened a mass media campaign accusing the United States of espionage, after acknowledging yesterday that one of its own fighter planes downed a South Korean jumbo jet last week.

A spate of official commentaries, readers' letters, and reports from foreign capitals in the press and on radio and television offered a picture of the Soviet Union as a victim of an American plot. The United States was also attacked for its actions in Lebanon and Central America.

The current Soviet press coverage of the plane tragedy contrasts with the virtual silence in Moscow initially.

Last night's government statement that Soviet air defenses had "stopped" the flight was interpreted by Western diplomats here as a concession to the furor raised in the West. Analysts attributed the delay to uncertainty within the Kremlin leadership over how to handle the issue, but predicted there would be little further change in the Soviet position.

The diplomats noted that the acknowledgment of technical responsibility for downing the plane came on the eve of the final session of the Madrid security conference. It could have proved embarrassing for Foreign Minister Andrei Gromyko to be confronted with evidence about the plane's fate in the form of taped conversations between the Soviet pilots and ground control.

The Soviet press comment today appeared designed to bolster several key Kremlin claims: that the South Korean 747 was on a spy mission for the United States, that it was impossible for Soviet fighter pilots to tell it was a civilian airliner, and that the Soviet Union was legally justified in shooting down any "intruder."

A cartoon in the government newspaper Izvestia this evening showed a caricature of an American spy carrying a camera and a telescope being prevented from entering the Soviet Union. The caption read: "No farther, mister."

The Soviet authorities have pointed to the fact that the South Korean plane overflew secret bases as evidence that it was on a spying mission. The government statement also claimed that it emitted coded signals of the type used in spy operations.

Several Soviet papers carried letters from readers supporting the way the Soviet planes had dealt with the matter. A soldier was quoted as saying: "The Soviet Union and its armed forces are always ready to give reliable protection to our sacred frontiers."

To support its contention that "indignation is rising around the world" against President Reagan, Tass quoted a leader of the U.S. Communist Party, James Jackson, as saying the Reagan administration had "launched intensive preparations for war against the Soviet Union." Soviet Media Continuing To Point Finger at U.S. By Michael Dobbs Washington Post Foreign Service

MOSCOW, Sept. 7--The Soviet Union today broadened a mass media campaign accusing the United States of espionage, after acknowledging yesterday that one of its own fighter planes downed a South Korean jumbo jet last week.

A spate of official commentaries, readers' letters, and reports from foreign capitals in the press and on radio and television offered a picture of the Soviet Union as a victim of an American plot. The United States was also attacked for its actions in Lebanon and Central America.

The current Soviet press coverage of the plane tragedy contrasts with the virtual silence in Moscow initially.

Last night's government statement that Soviet air defenses had "stopped" the flight was interpreted by Western diplomats here as a concession to the furor raised in the West. Analysts attributed the delay to uncertainty within the Kremlin leadership over how to handle the issue, but predicted there would be little further change in the Soviet position.

The diplomats noted that the acknowledgment of technical responsibility for downing the plane came on the eve of the final session of the Madrid security conference. It could have proved embarrassing for Foreign Minister Andrei Gromyko to be confronted with evidence about the plane's fate in the form of taped conversations between the Soviet pilots and ground control.

The Soviet press comment today appeared designed to bolster several key Kremlin claims: that the South Korean 747 was on a spy mission for the United States, that it was impossible for Soviet fighter pilots to tell it was a civilian airliner, and that the Soviet Union was legally justified in shooting down any "intruder."

A cartoon in the government newspaper Izvestia this evening showed a caricature of an American spy carrying a camera and a telescope being prevented from entering the Soviet Union. The caption read: "No farther, mister."

The Soviet authorities have pointed to the fact that the South Korean plane overflew secret bases as evidence that it was on a spying mission. The government statement also claimed that it emitted coded signals of the type used in spy operations.

Several Soviet papers carried letters from readers supporting the way the Soviet planes had dealt with the matter. A soldier was quoted as saying: "The Soviet Union and its armed forces are always ready to give reliable protection to our sacred frontiers."

To support its contention that "indignation is rising around the world" against President Reagan, Tass quoted a leader of the U.S. Communist Party, James Jackson, as saying the Reagan administration had "launched intensive preparations for war against the Soviet Union."