Security has been tightened recently around President Reagan and other top government leaders because of intelligence reports that Libyan assassination squads are trying to enter the United States, officials said yesterday.

One administration official, who declined to be identified, said increased precautions for the safety of Reagan and Vice President Bush have been in place for about two weeks. Secretary of State Alexander M. Haig Jr. and Secretary of Defense Caspar W. Weinberger also are being guarded more closely, sources said.

Another source said surveillance has been increased along the Canadian border, possibly between Detroit and Windsor, Ontario, because of intelligence reports that the suspected assassination teams might come through Canada. "It is something you just can't ignore," one Justice Department official said.

Another official said the FBI has heard unconfirmed reports about Libyan assassination teams "that are persistent enough that they can't be overlooked. It's sort of a Catch-22 situation. The information isn't that specific, but you take none of it lightly."

Wire service reports that one intelligence report from the Middle East last week included the names of six would-be killers have not been corroborated, the official said.

Libyan officials have denied allegations about an assassination campaign against U.S. officials, dismissing them as administration propaganda.

The administration has made a foreign-policy issue of Libyan leader Col. Muammar Qaddafi and his attempts to export terrorism.

Haig suggested earlier this month that Qaddafi may have been behind the attempted shooting of Christian Chapman, acting U.S. ambassador in Paris.

When neither Reagan nor Bush attended the funeral of slain Egyptian President Anwar Sadat last month, reports cited other intelligence information about possible Libyan assassination squads.

Recent intelligence reports have been given credence because of the assassinations of several of Qaddafi's political opponents in Europe and the attempted murder of another Libyan dissident in Colorado.

Eugene A. Tafoya, a former Green Beret, is on trial in Fort Collins, Colo., for the latter shooting. but rulings by the judge have prevented the jury from hearing evidence of Tafoya's connections with former Central Intelligence Agency officer Edward P. Wilson, indicted for exporting terrorist equipment to Libya.

An ABC report said FBI agents have been pulled off other cases and assigned to a special counterintelligence group to concentrate on finding Libyan agents in the United States.

But an FBI official said last night that there has been no major shift of agents. And a Justice official agreed that the heightened security was a precaution, not a reaction to specific information that a band of foreign terrorists is roaming the countryside.