In the three weeks since U.S. warplanes struck at Libya, the Reagan administration's contention that Libyan leader Muammar Qaddafi supports international terrorism has been buttressed by statements from six West European governments alleging Libyan complicity in a variety of terrorist plots.

These charges are in addition to what U.S. officials describe as a string of more than 20 instances of terrorism -- some enacted, others thwarted -- since November. Collectively, the evidence sketches an apparent shift by Qaddafi from attacks aimed mainly at Libyan dissidents to a far more extensive assault on Western interests throughout Europe and the Middle East, according to U.S. sources.

U.S. officials said the shift started to become apparent with the Nov. 23 hijacking of an Egyptian airliner to Malta and the Dec. 27 attacks at the Rome and Vienna airports, followed by the April 5 bombing of a West Berlin nightclub. The officials said that they have evidence tying Libya to all three incidents, and President Reagan cited "incontrovertible evidence" of Libya's complicity in the Berlin bombing as the reason why he ordered the April 15 air strikes against Qaddafi.

Since then, several West European leaders who had been reluctant to speak out against Qaddafi have come forward with their charges against Libya:

*West German Chancellor Helmut Kohl, citing his government's independent investigation, backed up the U.S. charge that the Libyan People's Bureau in East Berlin had ordered the nightclub bombing that killed a U.S. soldier and a Turkish woman and injured 230 others. Both Kohl and Reagan are understood to have based their charges on the decoding of an intercepted message between Tripoli and the Libyans' East Berlin mission.

*French officials corroborated statements by Reagan and Secretary of State George P. Shultz that they thwarted a Libyan-inspired attack, planned for March 28, on persons waiting in line for visas at the U.S. Embassy in Paris.

*The Italian government charged that a former Libyan diplomat was involved in a plot last year to murder the U.S. ambassador in Rome.

*The chief state prosecutor in Turkey announced that two Libyans had been arrested on charges that they conspired to blow up an American officers' club in Ankara with explosives supplied by Libyan People's Bureau personnel.

*British Foreign Secretary Geoffrey Howe said that two kidnaped Britons, who were found murdered near Beirut on April 17, had been killed by Libyan agents.

*Spain expelled 11 Libyans, including a diplomat, on the grounds that they had been involved in threats against Spanish security.

In most cases, these countries have not revealed the evidence underlying their charges. But the specificity of their accusations marked a significant change from the time when the United States stood virtually alone in denouncing Qaddafi as a supporter of terrorism; U.S. officials say that the pattern of his activities has been known to U.S. and allied intelligence services for some time.

Originally, the U.S. sources said, Qaddafi concentrated largely on trying to kill dissident Libyan exiles and to foment subversion in neighboring nations such as Egypt, Sudan and Chad. In the wider sphere of international terrorism, the sources said, Qaddafi long confined himself primarily to contributing funds to groups such as the Palestine Liberation Organization and the Irish Republican Army.

However, the sources continued, that pattern began to change in early 1985 when the terrorist group headed by Abu Nidal, a renegade from the PLO, became allied with Libya. That reportedly gave Qaddafi the ability to extend the reach of his terrorist activities. U.S. officials, while declining to give details, contend they have strong reason to believe that the Abu Nidal group, at Libya's behest, carried out the Malta plane hijacking in which 59 passengers were killed and the December airport attacks that left 19 dead.

Since then, the sources said, the charges made by European governments and other intelligence amassed by the United States indicate that Libya not only is financing and directing terrorists such as Abu Nidal, but also is using its own agents, usually working behind the diplomatic cover of its embassies, to play the kind of direct role in terrorism that has been alleged against Libyan diplomats in Berlin, Paris, Rome and Ankara.

In describing other terrorist acts of recent weeks, such as the murder of a kidnaped American librarian in Lebanon, the officials said that in some cases there is insufficient evidence to categorize Libyan involvement as anything more than a strong possibility.