Mass demonstrations were reported yesterday and today in principal Libyan cities to protest the U.S. downing of two Libyan jets in the Gulf of Sidra, but Col. Muammar Qaddafi went about his business as if nothing had happened, flying from South Yemen to Ethiopia to begin a previously planned state visit.

The official JANA news agency cited protests yesterday in the capital, Tripoli, and the port city of Benghazi on the Mediterranean. It said "the popular masses" had staged the apparently peaceful mass rallies "to show their anger and challenge the aggressive American acts."

Radio Tripoli said the demonstrations shifted today to the town of Misratah, at the mouth of the Gulf of Sidra, and the desert city of Sebha.

A Libyan diplomat in Paris said the U.S. citizens in Libya, estimated at 1,500 to 2,000 oil workers and their families, would not be held responsible for the acts of their government.

Meanwhile, Qaddafi ended a meeting with the heads of state of South Yemen and Ethiopia in the Yemeni port of Aden and flew off to Addis Ababa with Ethiopia's leader, Col. Mengistu Haile Mariam. The Ethiopian news agency reported the two leaders were greeted by half a million people.

Qaddafi remained silent about the downing of his planes by U.S. F14s flying maneuvers 60 miles of the Libyan shore in waters claimed by Libya but considered by the United States as international waters. The two countries differ on territorial control of offshore waters, with the United States recognizing only three-mile limits and Libya claiming 12 miles. More pertinent to this case, Libya claims the entire Gulf of Sidra as its water, drawing an east-west territorial line between two northern promontories of the country instead of directly following the indentation if the coastline.

While Arab editorialists from the Persian Gulf to North Africa denounced the U.S. naval maneuvers off Libya, calling them acts of "provocation" and "piracy," Qaddafi did not even mention them in the speech he delivered in Aden Wednesday afternoon at the close of the trilateral summit -- which sought to forge an anti-Western axis along the shores of the strategic Red Sea.

There was speculation among analysts of Arab affairs here that he would wait until his return to Libya to denounce -- and possibly act against -- the United States. The 12th anniversary of the revolution that brought Qaddafi to power comes Sept. 1. The Libyan leader generally reserves it to make major pronouncements and he may wait until he mobilizes the population for those festivities before officially reacting.

Qaddafi's arrival in Addis Ababa coincided with a stiff attack on the United States by the Ethiopian government.

"The arrogant and recklessly adventurous Reagan administration has let it be known that it intends to continue carrying out acts of international terrorism and brigandage to impose its will," the Ethiopian Foreign Ministry said. "The United States has created a dangerous situation that will have grave consequences for international peace."

Syrian Foreign Minister Abdul Halim Khaddam telephoned Libya's deputy leader, Abdul Salim Jalloud, to say his country, which only a year ago had announced a "merger" with Libya that has not been heard about since, pledged its full support to Libya in the confrontation with the United States.

According to a spokesman from the Syrian Foreign Ministry, Khaddam told Jalloud "any aggression against Libya is an aggression against Syria" and Syria was prepared to "carry out whatever measures are required by brotherly and nationalist obligations."

Libya lodged an official complaint with the U.N. Security Council, United Press International reported, that accused the United States of a "hideous act," with eight fighters attacking two of its warplanes and shooting one of them down. The letter from Libya's nominal foreign minister, Abdulati Obeidi, to the council president did not request any action by the U.N. body. On Thursday, a Libyan diplomat in London had acknowledged loss of two planes, in accord with the U.S. version of the battle.

According to a Libyan news service report, Libya today refused to accept a U.S. diplomatic note protesting the clash as the result of an unprovoked attack by Libyan planes, UPI said. The note was presented to the Libyan government by a Belgian diplomat on behalf of the United States.

Associated Press reported that Jordan's leading newspaper, Al Rai, said: "The American piracy against Libya is a form of international terrorism and a unique form of piracy unknown by the world and previously only carried out by organized criminal gangs. While there was no official comment, Jordanian newspapers are closely controlled by the government.

Arab League Secretary General Chadli Kleibi denounced the U.S. naval maneuvers off Libya as "a violation of the peace and stability of Arab nations" and said they "can only increase tension in the Middle East area."