U.S. warplanes yesterday attacked a Libyan antiaircraft site and two guided missile patrol boats after Libya fired at least four SA5 missiles at Navy aircraft that had crossed "the line of death" drawn by Muammar Qaddafi across the Gulf of Sidra, White House and Defense Department officials announced.

The Pentagon said no American planes were lost in the attacks despite Libyan claims that three U.S. aircraft were downed. Defense officials said they had no estimate of Libyan casualties, although one of the two patrol boats was burning and appeared to be sinking. Pentagon officials said it is not known whether any Soviet technicians were at the antiaircraft site during the raid, which occurred at 9:06 p.m. Libyan time (3:06 p.m. EST).

An A7 bomber from the aircraft carrier USS Saratoga fired "one or two" High-speed Anti-Radiation Missiles (HARM) at two Soviet "Square Pear" radars at the Libyan antiaircraft site at the coastal town of Surt, according to Defense Secretary Caspar W. Weinberger, who briefed reporters on the raid. Those radar installations controlled the long-range SA5 missiles fired at Navy aircraft in sporadic attacks beginning at 7:52 a.m. EST, according to the White House.

Five Libyan SA5s were fired at Navy fighter planes orbiting above U.S. warships in the Gulf of Sidra beyond the 150-mile range of the missiles, a Pentagon official said. The missiles fell harmlessly into the water, far short of their targets.

The attack against Surt was launched after the fifth SA5 missile was fired at 1:14 p.m. EST, according to the White House. A sixth missile, a shorter-range SA2, was also fired but missed its target. (It was not clear last night precisely how many missiles had been fired; estimates ranged from four to six.) The Navy planes attacked the Libyan radar positions less than two hours later.

"This site is out of action now," Weinberger said. "When hostile acts are committed against you, why then you respond."

Officials said they did not have certain information about how much the SA5 radars were damaged in the HARM attack, except that they were no longer operating. They said they thought that there were no casualties at the site, however. If the Libyans manage to return their radars to service, Pentagon sources said last night, the Navy has plans to launch another raid.

On Capitol Hill, there was generally bipartisan support for the U.S. raid. Neither White House nor Pentagon officials described the incidents as acts of war, and there was little immediate reaction from U.S. allies and adversaries abroad.

U.S. authorities acknowledged concern about a Libyan retaliation against U.S. installations or citizens abroad, or direct terrorist attacks in the United States, as Qaddafi has vowed. Estimates on the number of U.S. citizens remaining in Libya despite a recent presidential order to leave range between 100 and 1,000.

In a broadcast over state radio last night, the Libyan government threatened to "strike mercilessly" against the U.S. Sixth Fleet and "make the Mediterranean into a sea of fire." Earlier yesterday, in a letter to the United Nations, the Libyans warned that the U.S. military exercise in the Gulf of Sidra gave them the right to strike back against any armed attack.

Questioned about the alleged Libyan provocation, Weinberger said, "We were thoroughly and completely satisfied after numerous corroborative evidence and sightings and other things that SA5 missiles were fired."

Although Weinberger did not elaborate, it is standard practice for a converted transport plane full of electronic eavesdropping equipment and manned by technicians fluent in Arabic to be aloft during risky U.S. naval operations. The plane, code-named Burning Wind, works under direction of the National Security Agency.

The attacks on the Libyan high-speed patrol boats were made by A6 bombers armed with antiship Harpoon missiles and Rockeye cluster bombs, according to military sources. The first boat struck, the Pentagon said, was speeding eastward out of the port of Misratah and appeared intent on firing at Navy warships sailing inside Qaddafi's "line of death" at 32 degrees 30 minutes north latitude, which extends more than 100 miles into the U-shaped Gulf of Sidra.

The United States had served notice that it would not honor that line; the U.S. Navy also publicly reserved the right to fly and sail within 12 miles of the Libyan coast, the internationally recognized limit on territorial waters. Navy planes crossed the "line of death" on Sunday and warships sailed across it yesterday, the Pentagon said. But the three aircraft carriers in the task force -- the USS America, USS Coral Sea and USS Saratoga -- were held outside Qaddafi's forbidden zone.

Weinberger said one of the Libyan missile patrol boats got within 38 miles of an American warship, putting the U.S. ships within range of Libyan missiles. The defense secretary was vague as to how close the French-made Combattante II class boat came before it was attacked by A6 bombers from the America.

The A6s flew from the America armed with Harpoon antiship missiles. At 3:06 p.m. EST, Weinberger said, the bombers hit the patrol boat with two Harpoons, the first combat use of the antiship weapon by the United States.

"That ship was left sinking and on fire," Weinberger said. "We saw no survivors from it."

Almost an hour later, at 4:19 p.m. EST, a second Combattante sped westward out of Benghazi with apparent hostile intent, Weinberger said. Other A6 bombers attacked it with Rockeye bombs. "The first reports were that it was severely damaged," Weinberger said.

Weinberger, who had opposed earlier plans to bomb Lebanon for fear of inciting the Arab world, indicated no such reluctance in justifying yesterday's attacks.

"All of these are actions that we took because our announced naval and air exercises are in international waters and were seriously interfered with by evidently hostile acts that could have caused great damage to our ships, exercising and maneuvering in areas where they're entitled to be," the secretary said.

White House spokesman Speakes warned that there may be further attacks on Libyan forces if they try to interfere with the Navy operations in the Gulf of Sidra. "We now consider all approaching Libyan forces to have hostile intent," Speakes said.

The United States over the weekend issued a formal notice of intent to conduct Navy flight operations in the Gulf of Sidra region until 6:59 p.m. EST on April 1. Weinberger said the big task force of three aircraft carriers, 27 escorting warships and more than 240 planes massed off Libya might break off operations before that time, but he indicated they will continue for at least the next several days.

Because of concerns that Qaddafi could authorize retaliatory attacks against U.S. Navy leaders, extra armed guards from the Naval Investigative Service were visible last night in the office of Adm. James D. Watkins, chief of naval operations. Navy Secretary John F. Lehman Jr. was expected to be assigned extra guards as well.

Yesterday's strikes against the Libyan missile site and patrol boats were elaborately planned weeks ago, according to Pentagon sources, even to the point of briefing pilots on the best way to knock out the Surt complex and filing the details in what was called a "ready strike" folder. However, implementing the strike plans depended on some overt hostile act by Qaddafi, military sources said.

Military critics of the Pentagon's recent planning considered the massing of three aircraft carriers off Libya to be overkill. There were suspicions, even at the Pentagon, that hawks in the administration were deliberately trying to provoke Qaddafi into taking some act that would justify retaliation.

"Now what do we do?" said one military critic, who predicted that Libyan-sponsored terrorist attacks in the United States will follow.

Military backers of yesterday's strikes countered that a superpower such as the United States could not allow Qaddafi to deny it use of international waters.

Weinberger and other administration officials had been saying for the last several days that they were not trying to provoke Qaddafi, but rather exercising the U.S. right of navigation in the Gulf of Sidra. Speakes said the United States showed forbearance before ordering the first set of retaliatory actions.

While denouncing Qaddafi's government as "an outlaw regime and up to no good," Speakes said "we were there" in the Gulf of Sidra "on a peaceful exercise . . . . I can't characterize it as war."

After the first two SA5 missiles were fired, Weinberger said, Libya sent out two Soviet MiG25 aircraft to challenge Navy planes. The Mig25s were turned back. Then two more SA5 missiles were fired, Weinberger said.

"After that," he said, "why then it becomes quite clear we have to take the necessary actions to protect our units where they are."

The Soviet Union had helped Libya build its antiaircraft site at Surt. An estimated 12 launchers there had become operational, according to U.S. sources. The U.S. intelligence community believed that Qaddafi had insisted on deciding when the SA5s would be fired rather than receiving advance approval from his Soviet advisers.

After terrorists attacked the Rome and Vienna airports on Dec. 27, the Joint Chiefs of Staff drew up a list of potential targets in Libya for President Reagan. The targets included the Surt antiaircraft missile site, sources said. But the chiefs saw no linkage between the site and the terrorists, who Reagan said were sponsored by Libya.

Reagan and the chiefs have searched for tit-for-tat kinds of military responses ever since the target list was rejected for lack of linkage. Using Navy planes to bomb the same site that fired on other Navy planes fit the Pentagon's requirement for appropriate responses.

The weapons fired at Libyan radars represented an advance in the use of missiles for waging naval warfare rather than the old-fashioned gravity bombs used exclusively before the Vietnam war.

The HARM is a 23-foot-long missile that electronically homes in on the radar beams from an antiaircraft site, destroying it with a high-explosive warhead. It is a third-generation antiradar weapon, which has had persistent cost and performance problems in the past but seems to have worked well yesterday in destroying the Soviet-made radars guiding the SA5s.

The Harpoon antiship missile used against at least one of the two Libyan patrol boats is 15 feet long and emits radar signals to detect a profile of the target. When the radar image matches one stored in its mechanical brain, the Harpoon dives into the ship, where its 500-pound warhead explodes.

Navy bombers, while standing off Lebanon in 1983 and 1984, were armed mainly with cluster bombs to destroy attacking high-speed boats like the French-made Combattante deployed by Libya yesterday. The Combattante ordinarily carries a crew of 27.

Neither the Libyan air force nor navy is experienced in night operations, which the U.S. Navy practices constantly. Officials said much of yesterday's action was conducted in the dark to exploit that American advantage.

An even bigger advantage enjoyed by the the U.S. Sixth Fleet standing off Libya, military officials said, is its surveillance capabilities. Aegis cruisers with the task force have the latest radars to detect almost every move Libyan military forces make. The carriers also carry EA6B Prowler electronic eavesdropping planes, which can tell when Libyan forces are put on alert, and E2C Hawkeye command-and-control aircraft, which can track a Libyan plane from takeoff and guide U.S. planes to intercept it.

If the Libyan air force should engage Navy planes, they would be heavily outgunned. The armament of Navy planes includes an infrared AIM9L Sidewinder missile, which can hit an enemy plane from any direction, including "face to face." In order for Libyan pilots to use their heat-seeking missiles effectively, they must position themselves near the rear of the target plane so the missiles can sense the exhaust heat.