Canada ordered tighter surveillance of all flights entering and leaving the country yesterday as authorities launched wide-ranging investigations into the crash of an Air-India Boeing 747 in the Atlantic and an explosion in Tokyo that killed two airport workers unloading luggage from a flight that originated in Vancouver.

Canadian officials, disclosing that they had received a formal request from India's diplomatic mission in Canada for extra protection for Indian diplomats and Air-India flights about a month ago, indicated that they were focusing the investigations on the possibility that radical members of the Sikh religion living in Canada may have been involved in yesterday's incidents.

"Although we have reached no conclusions as of this date, we are examining the possibility that the two incidents have some connection," said Canadian Foreign Ministry spokesman Sean Brady.

India's diplomatic mission in Canada reported to the government about a month ago that it had received threats against its envoys and national airline's flights that were thought to be from Sikhs.

The request triggered a number of special precautions, including the posting of Royal Canadian Mounted Police at check-in counters and embarkation areas for Air-India.

Until recently, Canadian officials said, searches had involved the use of guard dogs trained to detect explosive devices. But Air-India officials, reportedly concerned that this procedure was deterring passengers from taking the airline, began using X-ray equipment to inspect registered baggage.

This could have played a role in the Air-India disaster yesterday off Ireland. The X-ray machines in Toronto, where the flight originated, broke down Saturday night, and personnel resorted to hand searches, Canadian officials said.

Flight personnel did prevent three pieces of luggage from being loaded on the plane when it stopped next in Montreal to pick up passengers. When those suitcases were examined yesterday, nothing suspicious was found in them, officials said.

Late yesterday, The New York Times informed the Canadian Foreign Ministry in Ottawa that the newspaper's New York office had received an anonymous local telephone call from a man speaking with a heavy accent who said a group called the "Sikh Student Federation" had been responsible for the Air-India crash. He said they were protesting "Hindu imperalism."

Canadian authorities said last night that the federation, an extremist group based in India, has a significant number of supporters in Canada, especially around Vancouver.

The overall leader of the Sikh extremists, Jarnail Singh Bhindranwale, was killed a year ago during the Indian government's raid on the Golden Temple in Amritsar, the holiest site of the Sikh religion. The attack prompted a wave of extremist Sikh threats, culminating last October in the assassination of Indian prime minister Indira Gandhi by Sikh bodyguards.

In Canada, factional struggles have occurred in the Indian community, estimated to total more than 200,000, following the raid on the Golden Temple. Last July, a high-ranking Indian diplomat was roughed up by Sikh militants in Winnipeg, and there have been other instances of radical groups storming consular offices and threatening moderate members of the Sikh and Hindu communities.

Two years ago, factional strife resulted in two Sikhs being killed in a shooting in a Toronto courtroom.

Canadian Foreign Ministry and aviation officials met throughout yesterday in their "emergency crisis center" in Ottawa, as they began investigating the airline incidents and ordered special security measures for all flights originating from airports in Canada to Asia, Europe and Africa.

Aviation Minister Don Mazankowski requested that foreign governments undertake similar measures for flights headed to Canada. Canadian Foreign Minister Joe Clark expressed condolences in a statement yesterday for those who had been killed.