President Reagan and his top advisers made an extraordinary disclosure of sensitive intelligence information last night to demonstrate that the United States had hard evidence that Libya not only was directly responsible for the bombing of a West Berlin nightclub 10 days ago, but also was planning attacks against U.S. diplomats, dependents and businessmen abroad.

The specifics cited by the president last night, sources said, will make it clear that the United States has the capability to intercept and decode Libya's sensitive diplomatic communications.

Sources said the decoding was done by the National Security Agency (NSA), whose code-breaking capability traditionally has been one of the most closely guarded intelligence secrets.

Regarding the attack in Berlin, Reagan laid out the U.S. case for Libyan culpability by citing three messages that other sources said were intercepted diplomatic cables. The first, he said, was a March 25 message from Tripoli to the Libyan People's Bureau in East Berlin directing a terrorist attack on Americans. This was the day after U.S. planes attacked Libyan missile sites and sank two Libyan patrol boats in the Gulf of Sidra after being fired upon by Libyan military forces.

On the evening of April 4, Reagan said, the Libyan bureau in East Berlin reported back to Tripoli that an attack would take place the next day.

It did. A bomb exploded at a crowded West Berlin discotheque, killing an American serviceman and wounding more than 200 persons.

A third message from the Libyan bureau came after the attack, the president said. The message reported the "great success of the mission," he told a national television audience.

White House spokesman Larry Speakes last night told reporters that in this final message, the bureau assured Tripoli that the terrorist operation had gone undetected and could "not be traced to the Libyan People's Bureau."

In referring to these Libyan messages, Reagan said, "Our evidence is direct, it is precise, it is irrefutable."

Intelligence sources said this week that the timely intelligence paints a remarkably clear picture of the terrorist operation and that a jury in an American courtroom would almost certainly convict Libya. At the same time, the sources said the intelligence does not absolutely show that Libyan leader Muammar Qaddafi had a personal and direct hand in the secret terrorist planning.

Reagan himself last night stopped short of tying Qaddafi personally to the West Berlin disco bombing. Reagan said that terrorist operation "was planned and executed under the direct orders of the Libyan regime."

"There's no question that Qaddafi knew what was going on," said one intelligence community official who asserted that "nothing gets done" in Libya without Qaddafi's approval.

Sen. David F. Durenberger (R-Minn.), the chairman of the Senate Select Committee on Intelligence, said he had been shown the evidence linking Qaddafi and Libya to the West Berlin bombing and found it convincing.

"In my judgment, it is enough for the president to go on," he said. "There wasn't any question in my mind or that of the committee's professional staff as to the linkage."

The president made another extraordinary disclosure when he revealed that the United States "with the help of French authorities recently aborted" a Libyan attack. Without naming the country, Reagan called it a "planned massacre using grenades and small arms" and targeted at "civilians waiting in line for visas at an American embassy."

According to U.S. officials, the two Libyan diplomats expelled from France this month were allegedly planning an attack on the U.S. consulate in Paris at a time when visa applicants would be standing outside and vulnerable to attack from the street.

In stressing that last night's strike was intended to deter Libya from further terrorist aggression, the president recited a number of plots uncovered by U.S. and allied intelligence services targeted at American embassies, diplomats and their children, American businessmen, their companies and at U.S. tourists in Europe, the Middle East, Africa and Latin America.

U.S. officials have asserted that Libya's stepped-up terrorist planning began late last year, but one well-placed intelligence source said that most of the increase followed last month's Gulf of Sidra confrontation. This source voiced concern that the United States and Libya were entering a cycle of escalating violence.

Speakes and other officials said last night that after the Gulf of Sidra clash, Libyan operatives throughout Europe, Africa and the Middle East went into action in search of targets for retaliation.

"We do have highly reliable intelligence that Qaddafi and his key lieutenants are planning more attacks on U.S. citizens and facilities in Europe, the Middle East and Latin America," said Speakes. "In Africa," Speakes continued, "Libyans have been planning attacks and conducting surveillance on U.S. facilities in 10 countries."

In a reference to one of these, Speakes said, "Last week, three Libyan agents arrived in one African state to set up the bombing of our embassy, chancery and the kidnaping of our ambassador."

One intelligence source said last night that an attack similar to the one Speakes described was detected and aborted in the Central African Republic and a description of it, along with the French role in uncovering it, was circulated in a highly classified intelligence report yesterday.

Sources said U.S. intelligence supplied to West German officials about a pending attack on the consulate in Munich triggered the expulsion of two Libyan diplomats this month.

One source said the U.S. government has made a request to the East German government to close the People's Bureau there because of the alleged role of its Libyan officials in directing the West Berlin disco bombing. This source said the East Germans asked to see the U.S. evidence before acting, but U.S. officials declined to turn it over.

In the last several years, U.S. intelligence agencies have so increased intelligence gathering targeted at Qaddafi that the sources said there are dozens of high-grade intelligence reports showing that Qaddafi has encouraged and authorized terrorist acts as a matter of policy.

The U.S. capability to penetrate foreign diplomatic and military communications is the responsibility primarily of the NSA, which operates a worldwide communications surveillance network including ground listening stations, spy ships and satellites. Once intercepted, foreign radio, telephone and other communications can sometimes be decoded with the assistance of high-speed computers.

The electronic "take" from these sophisticated listening stations is translated into top-secret intelligence reports that often carry additional classification levels to "compartmentalize" them for access only by a relatively small number of administration officials who have a "need to know."