An Air-India Boeing 747 en route from Montreal to London crashed into the Atlantic off the Irish coast this morning, apparently killing all 329 passengers and crew. Indian government and airline officials said they suspected sabotage, and initial indications pointed to a bomb aboard the plane.

By nightfall, a massive international air and sea rescue operation had recovered more than 100 bodies floating along with wreckage about 110 miles off southwestern Ireland. British Coast Guard officials coordinating the search at Falmouth, on England's southern coast, said they did not expect to find any survivors.

The crash came less than an hour after another airline incident halfway around the world at Tokyo's Narita International Airport, where an explosion in baggage taken from a Canadian Pacific Airlines flight killed two airport workers and injured four others.

The timing of the two events, and the fact that both planes had taken off from Canada, the CPA flight from Vancouver, led to speculation, but no initial evidence, that they could be related. Today's events follow the explosion of a bomb last week in the main terminal of the Rhein-Main International Airport in Frankfurt, West Germany, in which three persons were killed.

In the Air-India crash, a British Coast Guard officer said the rescue effort, in which a number of U.S. Air Force planes stationed here were participating, was "not so much a question of a search as just picking things up."

All of the crew was Indian, and nearly all the passengers were believed to be of Indian origin. About 280 of the passengers reportedly were Canadian passport holders while six U.S. residents were believed to be aboard the flight. It had originated in Toronto, with scheduled stops in Montreal, London and New Delhi en route to Bombay.

Callers purporting to represent an Indian and a Pakistani group claimed responsibility for destruction of the plane. New York Times news aide Patricia Ochs said a man speaking with a heavy accent, in what seemed to be a local call, said the Sikh Student Federation acted "to protest Hindu imperialism." A caller to The Washington Post said the Kashmir Liberation Front was responsible and would bomb other flights until the Indian Kashmir region was ceded to the Islamic Republic of Pakistan.

Assuming no one is found alive when the search resumes at first light Monday, the crash was the worst airline disaster at sea, and the third-largest loss of life in aviation history.

Canadian officials said they would investigate both of today's incidents, and said that security at all airports would be tightened.

Indian Prime Minister Rajiv Gandhi dispatched government and airline officials to Ireland and Britain, and asked Japan for information from its investigation of the explosion at Narita Airport.

India has jurisdiction in the investigation of the Air-India crash because it occurred over international waters. The U.S. National Transportation Safety Board has been asked to participate in the probe, and a representative from the Federal Aviation Administration will be on its team.

Although Indian and Canadian officials appeared to choose their words carefully in statements today, they clearly were centering their investigation on the possibility that a bomb was placed aboard Air-India Flight 182, either in Toronto, or during its stop in Montreal. Three "suspicious" suitcases destined for the Air-India flight were intercepted during a search but were pronounced harmless by officials later in the day.

A number of aviation experts and officials from the British airline pilots' association echoed a statement by Flight International magazine editor Mike Ramsden, who said "a bomb is the most likely reason for the catastrophe."

Contributing to this belief, they said, was the fact that there had been no emergency call from the aircraft, which had been flying at 31,000 feet. Even if it had lost all four of its engines, aviation experts said, the plane would have glided for at least half an hour before crashing.

Instead, the wreckage was found in relatively small pieces spread over a circle 4.5 miles wide and centered almost directly beneath the plane's last radar coordinates, and air traffic controllers said it disappeared from their screens and seemed to have "dropped like a stone."

Air-India and Canadian officials today reported unspecified new threats to the airline and to Indian diplomats during the past month and said that security had been stepped up before today's incidents.

Sikh extremists seeking an independent state in India have mounted a campaign of violence against the Indian government during the past five years and have hijacked several domestic Indian flights in the past year.

Sikhs, who make up only about 2 percent of the population of India, are followers of a religion that broke away from Hinduism three centuries ago. They are easily identifiable by their turbans and beards, which they wear wound into a cord under their chins. Like Moslems, they believe that dying in the name of their religion will make martyrs of men. Radical Sikh elements have been fighting for an autonomous Sikh homeland since before India won its independence from British colonial rule in 1947.

There is a large Indian community in Canada, centered in Toronto and Montreal, and the Sikh community in Toronto long has been engaged in factional disputes.

Here in London, Jaggit Singh Chohan, a Sikh exile leader, said, "If this is sabotage, I condemn it. This is a very barbaric and heinous crime. We would never believe in such things, and no Sikh would ever dream of killing innocent people like this."

In separate telephone calls to The Washington Post, the Sikh Association of America and the World Sikh Organization condemned violence against innocent passengers. In a statement, the World Sikh Organization branded the phone call by the Sikh Student Organization claiming responsibility for the crash "the latest attack in the continuing campaign of defamation of the Sikhs by attempting to brand them as terrorists in the eyes of the western world."

The Air-India flight was scheduled to stop for refueling at London's Heathrow Airport, where only airline employes were scheduled to get off or on the plane before it continued to India.

The plane had taken off late from Toronto Saturday night, and was again delayed in Montreal, where the three suspicious bags were detected. At about 3 a.m. EDT, 45 minutes before the flight was due at Heathrow, the crew was in radio contact with air traffic controllers at Shannon, Ireland.

At 3:13, one Shannon controller told television interviewers tonight, the plane "just disappeared off the scopes. We tried calling him on the frequency, but there was no reply. There were no emergency calls."

Within a few minutes, an emergency radio transmitter on the plane, designed to activate in the event of a crash, began to emit signals at approximately the same radar coordinates from which the pilot had last communicated. Although closest to the Irish coast, the site was within a zone assigned to Britain under international rescue agreements.

The first vessel on the scene was a merchant ship, the Laurentian Forest. Within 90 minutes, Coast Guard officials said, a massive rescue effort had been launched. Eventually, it included six Royal Air Force helicopters and three Nimrod reconnaissance planes plus aircraft from the Irish Air Command, two Irish warships, one British warship, and Coast Guard vessels from both countries.

Two U.S. Air Force HH53 helicopters and two C130 Hercules cargo planes joined the effort. All merchant ships in the area were called to help, and Spanish and French fishing vessels in the vicinity participated.

Pilots and ships' captains on the scene said the wreckage was in numerous pieces, the largest about 10 yards across.

According to Coast Guard Senior Watch Officer Colin Sturman, interviewed in Falmouth by telephone, "There is very, very little, if any, chance of survivors. They are still picking them up. The priority is bodies, but we're also picking up wreckage" to assemble for the investigation.

Late tonight, there were uncon-firmed reports that a radio signal from the aircraft's "black boxes," the automatic recorders of flight information and cockpit voices, had been detected.

Search efforts were hampered by 15-foot swells and frequent squalls. Visibility at the site was estimated at four to seven miles, however. Although merchant vessels were released from service after dark, a Nimrod reconnaissance plane and naval vessels stayed at the site. Coast Guard officials said the helicopters would return to the site at 4 a.m.

British officials who will be assisting the Indian government in the investigation reportedly arranged for a survey ship with a minisubmarine to head for the crash site to search for the "black boxes."

"We are doing everything we can to locate the . . . recorders," said Geoffrey Wilkinson, chief accident investigator of the British Transport Department.

The bodies recovered today -- none wearing life jackets -- were picked up by both helicopters and ships operating at the site and taken for examination and identification to a temporary morgue set up at the airport in the city of Cork, on the southern Irish coast.

Prime Minister Gandhi, a former airline pilot for Air-India, said in New Delhi that he was "deeply shocked" by the crash, United Press International reported. "There is a terrible sense of loss," Gandhi said in announcing an inquiry. "My heart goes out in sympathy to the members of the bereaved families. May they bear this loss with courage and fortitude."