Libya's unpredictable leader, Col. Muammar Qaddafi, has threatened to send "hit squads" to assassinate his opponents overseas, reviving the 1980 executions in which at least nine opponents in exile were killed.

Western diplomats are concerned that Qaddafi, humiliated by two unsuccessful attempts to host an African summit, may return to adventurist schemes that led the United States to expel all members of Libya's diplomatic mission in Washington last year and urge Americans to leave the oil-rich country.

"We always thought chairmanship of the Organization of African Unity might be a moderating factor for Qaddafi," a veteran ambassador said. "The fact that he has not achieved his aim could drive him to more radical attitudes," after a period of relative calm.

In recent speeches, Qaddafi threatened "the liquidation" of antigovernment exiles and also intimated that he may be planning to use his Soviet-supplied weapons -- and perhaps even Libyan troops -- to intervene once more in the long-running civil war in neighboring Chad.

Speaking to a rally Oct. 7, Qaddafi said, "The Libyan Arab people would meet their full responsibilities of liquidating escaped agents of America." That is his way of referring to his overseas opponents, if, as one Western diplomat said, "he's fixated about the United States, which he sees as his implacable enemy."

Qaddafi said in his speech, which was little noticed overseas, "There shall be no mercy for the agents of America. The escaped hirelings, enemies of the Libyan people, shall not escape from this people."

He did qualify the threat slightly, saying that the People's Congress, scheduled to meet in January, "will take the final decision of implementing the liquidation."

"He could change his mind by then," one diplomat said, but that was the same method he employed in 1980, when four opponents were killed in Italy, two in Britain, and one each in West Germany, Greece and Lebanon. Other attacks failed, as well as attempts on the lives of some Western diplomats in Europe.

His speech "is a license to kill," said another diplomat. All diplomats interviewed declined to be identified.

European diplomats who complained to the government about Qaddafi's threat and warned that resumption of assassinations would strain relations were told by Libyan officials not to take this speech too seriously. None of the diplomats, however, was willing to do so.

"One dismisses his wilder threats at one's peril," an envoy said, noting that Qaddafi has a habit of carrying out such actions, no matter how outlandish they seem.

The threatened assassinations are probably directed closer to home than Europe. "This time the threat is aimed more at political opponents in countries like Sudan, Somalia and Morocco," a senior ambassador said.

Two of the targets, he said, are likely to be former Libyan ambassador to India Mohammed Youssef Lemgurief and Somalia-based Abdul Hamid Bakush, the next to last prime minister under King Idris, who was overthrown by Qaddafi in 1969.

Lemgurief is the secretary general of the National Front for the Liberation of Libya, whose anti-Qaddafi broadcasts from neighboring Sudan have enraged the Libyan leader. Lemgurief circulated a document at the United Nations recently criticizing Qaddafi's human-rights record and saying the Libyan leader was subverting the Arab world and African nations.

The "liberation" groups in Sudan and Somalia are part of the game of surrogate played in the region stretching from North Africa to the Horn of Africa, where many governments sponsor opponents of neighboring rulers.

For example, Qaddafi supports groups opposing Sudanese President Jaafar Nimeri and Somali President Mohammed Siad Barre.

Western diplomats think neither anti-Qaddafi group has much chance of persuading the Libyan populists that they are a valid alternative, since they are mainly backed by the old, powerless merchant class, which "never learned how to use weapons," as one source put it.

Recently there have been reports of an abortive coup plot by the Army that had been planned for Qaddafi's return from a trip to China, North Korea and Yugoslavia. British newspapers, citing Arab sources in London, reported that about 100 officers were arrested.

But diplomats, who acknowledge that they have little private access to Qaddafi, say there is no evidence of such a plot or arrest and the Libyan government newspaper Al Zahaf Al Akhbar denied it. There have been numerous rumors of coup attempts in the 13 years since Qaddafi took power.

Noting Qaddafi's humiliation at twice failing to convene the OAU's summit, a senior ambassador said, "Qaddafi's normal reaction would be to look for a new adventure," such as resumption of the assassination campaign.

"He is like a Bedouin planning to steal camels. He sits in his tent and reflects," said the ambassador. "All of a sudden he gets an idea and tries to do something.

"If he fails, he does not object too much. He goes back to the tent and looks for something else to do. He's a restless person."

Qaddafi may turn his restless eyes southward again to Chad, where in 1980-81 he used 15,000 troops and masses of armor and artillery purchased from the Soviet Union to maintain his protege, Goukouni Oueddei, in power. A year ago he pulled his forces out to placate the OAU before the scheduled summit, and Goukouni was overthrown by rebel leader Hissene Habre.

Last week Qaddafi told African leaders, "We have to support the Goukouni government because he has no guns."

Most analysts think Qaddafi will limit his support, at least for the time being, to providing weapons and training for Goukouni's forces since the previous Libyan involvement was unpopular at home, particularly with the military.

Qaddafi acknowledged losing 300 soldiers in the war. With a population of only slightly more than 3 million, that would be the equivalent of the United States losing more than 20,000 men in a war. There are some reports that the Libyan losses were much higher than 300.

"If he does go in, he will do it slowly," a diplomat said, "and probably first building up the excuse that Egypt, Sudan and other nations are arming and fighting for Habre." Such reports are already seen in Libyan newspapers.

Chad has not known peace and stability in more than 20 years of independence from France, but Habre is bound to have a hard time achieving stability. If he does stabilize Chad, however, he is bound to be regarded by Qaddafi as a threat to Libya.

With Habre in control of Chad, Libya would be surrounded on all sides, except Algeria, by hostile neighbors.