Navy warships last night prepared to leave the Gulf of Sidra after steaming closer to Libya's territorial waters, while Libyan leader Muammar Qaddafi announced intentions to demonstrate his own naval firepower elsewhere in the Mediterranean.

The Libyan government, according to maritime sources, telephoned shipping agents around the world yesterday to warn that Libyan warships planned target practice today in the area 100 miles north of Tripoli, far to the northwest of the contested Gulf of Sidra where American and Libyan forces have exchanged missile fire this week.

Shipping and oil industry officials predicted that if the U.S. 6th Fleet leaves the gulf area as expected by this morning, Qaddafi will take television crews to sea with him to watch the target practice and then declare to the world that he had chased the American fleet from his shores.

Even if the U.S. task force of three aircraft carriers and 27 escort ships extends its stay off the Libyan coast, the patch of ocean Qaddafi has picked for his naval demonstration apparently is more than 100 miles from the U.S. fleet around the Gulf of Sidra.

The Libyan navy consists of six submarines and about two dozen surface warships, including frigates and corvettes. In the skirmishes earlier this week, the 6th Fleet is believed to have sunk two Libyan patrol boats and damaged two others.

Pentagon officials said that Libyan military ships and planes yesterday continued to remain in Libyan territory. U.S. aircraft, listening with electronic eavesdropping equipment for the distinctive radar signals indicating that Libyan antiaircraft missiles are in a launch mode, did not detect the signals yesterday, the officials added. Thus U.S. officials saw no need for a third attack on the Soviet-made SA5 missile and radar site at the coastal town of Surt.

Navy A7 bombers from the USS Saratoga attacked the radars at Surt on Monday with High-speed Anti-Radiation Missiles (HARM), which home in on enemy radar beams. Pentagon officials said the Libyan radar dish was either replaced or repaired after the first attack, presumably by Soviet personnel who were believed to be inside the technicians' trailer at the antiaircraft complex.

A second attack was launched against the radars at 6:54 p.m. (EST) Monday, knocking out the fire control part of the Surt missile complex, according to defense officials.

Pentagon officials have told members of Congress that they do not believe any Soviets were killed at Surt during the attacks. To minimize what the military calls "collateral damage" that might have killed Soviets at the missile site, only the eyes of the antiaircraft system -- the radars -- were attacked, rather than the highly explosive SA5 missiles, six of which were said to have been fired at U.S. planes, provoking the retaliation.

Since the Gulf of Sidra appeared quiet yesterday, the Reagan adminstration's "Prairie Fire" plan of action for entering the contested gulf -- adopted at a secret March 14 meeting of senior national security officials -- remained in its first phase. The second and third phases of Prairie Fire, sources said, called for hitting Libya much harder.

If Libyan forces had killed American servicemen, phase two of Prairie Fire called for U.S. bombing raids on Libyan military targets, if so authorized by President Reagan, according to informed sources. A massive attack by the Libyans would have triggered phase three of Prairie Fire, in which bombing would have been extended to Libyan industrial targets.

There was confusion yesterday about how much damage the Libyan navy suffered during the fighting on Monday and Tuesday. On the basis of reports from the fleet, the Pentagon had said three Libyan patrol boats were sunk, two by aircraft and the third by two Harpoon missiles fired by the Aegis cruiser USS Yorktown.

A fourth Libyan patrol boat, the Pentagon said, had been damaged by Navy bombers. But officials yesterday acknowledged some doubt about whether the Yorktown's missiles had hit their target, and whether more than two boats had been damaged.

Pentagon spokesman Robert B. Sims declined yesterday to identify which warships had sailed across Qaddafi's so-called "line of death" across the mouth of the Gulf of Sidra at 32 degrees 30 minutes north latitude. Other officials identified the ships as the cruiser USS Ticonderoga, the guided missile destroyer USS Scott and the destroyer USS Caron.

Asked at the Pentagon briefing yesterday what had been accomplished in the unusually large three-carrier Navy exercise off Libya, Sims replied: "We have demonstrated that we have the right to operate in international waters. You have to do that periodically. There's nothing unusual about a freedom-of-navigation exercise. What is unusual is that Mr. Qaddafi chose to react to it with force."