Canadian Prime Minister Brian Mulroney today described the destruction Sunday of Air-India Flight 182 off the Irish coast and the explosion in luggage from a Canadian airliner in Japan as terrorism, but said investigators had not yet uncovered "the firm evidence" to back up his assertion.

Mulroney, at a press conference, described the two incidents as "cowardly and reprehensible acts." But when asked if his investigators were convinced that terrorists were behind both incidents, he said, "We do not have the firm evidence that would allow us to make that firm statement, but obviously we're very concerned about the fragmentary information that we have so far."

Investigators here have indicated they believe the two incidents were linked and possibly perpetrated by militants in Canada's Sikh community, who are in bitter opposition to the Indian government.

According to one informed source in Washington, Canadian authorities have detained three Indians, believed to be Sikhs, in Montreal for questioning in connection with the Air-India crash, which killed 329 persons.

Foreign Ministry spokesman Sean Brady said that the explosion in baggage from a Canadian Pacific Airlines jumbo jet in the baggage room of Japan's Narita airport, killing two baggage handlers, was "clearly terrorist," and that new evidence about the Air-India crash indicated that "it was a terrorist incident as well . . . .We are not discounting possible links between the two."

Investigators were reported to be focusing on "no shows" who bought tickets, checked baggage but did not board the two planes when they left Canada Saturday night.

Brady told the Canadian Press news agency that police were investigating an "individual who may have booked for the flight to Tokyo but not taken it, stopped off in Vancouver and not taken it, but had his baggage move on."

But both U.S. and Canadian authorities discounted a story in the Toronto Globe and Mail this morning alleging that two Sikh fugitives sought by the FBI in connection with a plot to assassinate Prime Minister Rajiv Gandhi during his visit to Washington earlier this month might be responsible for the two explosions.

U.S. officials acknowledged that both men had been sought by the FBI, but not apprehended, in New Orleans in connection with threats against Gandhi shortly before his U.S. visit. But Stanley Zuckerman, press spokesman at the U.S. Embassy here, said, "We have no information on which to base a conclusion that those individuals are in Canada."

Aviation authorities in Canada beefed up the numbers of Royal Canadian Mounted Police and customs officials at airports today, often subjecting travelers to rigorous searches.

At the Montreal airport, from which the Air-India flight left Canada after picking up passengers in Toronto on Saturday night, dogs capable of sniffing out weapons and explosives were carried around the airport checking lockers.

Transportation Minister Donald Mazankowski said an X-Ray machine at the Toronto airport had malfunctioned after about three-fourths of the plane's carry-on baggage had been cleared, and that the rest was screened with a hand-held metal detector.

Mazankowski said that if his department had been informed that three "suspicious" bags had been removed at Montreal the flight would have been held until all the luggage had been searched. The three bags were searched after the plane's departure but no explosives were found.

Mulroney, whose government faces stern questioning on airport security, said that its security arrangements "ranked with the most stringent in the world."

"Terrorism is the tragedy of our times," Mulroney said. "It is the most cowardly and reprehensible act known to man.""

Although Canadian authorities stressed that Air-India had employed procedures more stringent than other airlines in the wake of recent threats to the airline and to Indian diplomats, there was scattered private consternation among Canadian authorities that the airline may not have been tough enough in its precautions on Saturday night.

In the city neighborhoods and suburban communities of the more than 200,000 persons of Indian origin in Canada, there was both grief over the deaths in the North Atlantic on Sunday. There also was fear of a backlash against Indians here because of the suspicions that extremist Sikhs could be responsible for the disaster.

An alleged Sikh extremist has made a telephone claim of responsibility, but Sikh spokesmen have denied involvement of their community.