Libya said today its Air Force jets intercepted an unidentified foreign aircraft Wednesday off its coast near Benghazi and charged that U.S. reconnaissance planes stationed in Egypt were "jamming civil communications" inside its territory.

Libya seemed intent on playing up its latest confrontation with the United States, threatening through its official news agency to turn the Gulf of Sidra into "blood and fire" if American planes or ships sought to enter the disputed waters.

At the same time, the Libyan news agency reported demonstrations were held today in Tripoli and eight other cities, and the crowds burned effigies of President Reagan, Egyptian President Hosni Mubarak and Sudan's leader, Jaafar Nimeri.

The Jana news agency said the demonstrations were held "to protest the overt terrorist provocations of the U.S. Sixth Fleet" against Libya and denied that Libya was massing troops near the Sudanese border in preparation for an invasion as claimed by the U.S. government.

"We have not threatened the Sudan's territory and have not concentrated troops on its border," the official news agency said.

It also said that Libyan Air Force jets Wednesday intercepted "an aerial target" as it made an incursion "over our territorial waters" about 50 miles out to sea off the eastern coastal city of Benghazi.

The Jana report did not identify the nationality of the plane, but the U.S. government reported earlier this week that American jets from the aircraft carrier Nimitz had chased away several Libyan aircraft as they attempted to approach the ship in waters near the Gulf of Sidra. Thus, it appeared the report may have referred to the U.S. jets involved in this incident.

Jana also said that American Airborne Warning and Control System (AWACS) planes operating out of Egypt and U.S. Navy ships had begun "jamming civil communications" inside Libya Friday afternoon but gave no further details.

Libya claims the entire Gulf of Sidra as part of its territorial waters, but the United States does not recognize the claim. In August 1981, the two countries' warplanes engaged in a direct confrontation there, resulting in the downing of two Libyan jets.

The Reagan administration, however, is under increasing pressure from Egypt to ease the tensions and to avoid dragging this country into a Libyan-American confrontation.

The official Middle East News Agency issued a statement quoting "informed Egyptian sources" as saying Egypt "has never asked the United States to conduct any military moves in the Mediterranean or elsewhere."

The sources went to some lengths to disassociate Egypt from the simmering crisis, stating that it was "not an adherent to any country's strategy and has nothing to do with any moves by the United States or any other country" in the ongoing military maneuvers in the Mediterranean.

Egyptian officials went out of their way to play down the alleged threat from Libya to the regime of Sudanese President Jaafar Nimeri. It was reportedly Egyptian and Sudanese intelligence reports about a planned Libyan-backed coup against Nimeri that led the Reagan administration to send four AWACS planes to Egypt early this week to monitor Libyan military movements near the Sudanese border.

The Sudanese government has issued statements tending to confirm the seriousness of the threat.

The official Sudanese state radio in Omdurman said Libya was deploying air and ground forces at the southern Libyan oasis of Kufra and charged that "they are directed against the Sudan."

It said Sudanese security police had arrested members of a "revolutionary committee organization" that were allegedly planning to mount a coup attempt against Nimeri with the backing of Libya.

United Press International added the following: Francis West, assistant secretary of defense for international security affairs, arrived in Cairo Friday and was joined Saturday by Lt. Gen. Robert Kingston, head of the U.S. Rapid Deployment Force, the military command set up to protect western oil supplies and U.S. allies in the Middle East.

U.S. Embassy sources said West will attend the annual meeting of the U.S.-Egyptian military coordinating committee, a joint body set up to discuss long-range Egyptian planning and American military assistance.

In a related development, Egyptian officials said President Hosni Mubarak will travel to Khartoum on Tuesday to confer with Sudanese President Nimeri.

The officials said Mubarak's visit to Khartoum had been planned for some time and was not related to the reported Libyan threat to Sudan and Chad.

In Khartoum, the state-run Sudanese News Agency said Nimeri Saturday dissolved the capital's chapter of the Sudanese Socialist Union, the country's only legal political party.