VANCOUVER, OCT. 1 -- Canada has mounted a massive security effort to protect Indian Prime Minister Rajiv Gandhi during the Commonwealth conference here, because the city is the home of militant Sikh groups hostile to his government.

When Gandhi's plane landed Monday, helicopters buzzed overhead and Army riflemen swept across the tarmac. The airport formalities were brief, and Gandhi and a few members of his party were hustled into one of a fleet of eight identical black limousines, some apparently decoys.

Officials have told reporters that there are more than 3,800 police and soldiers here from across Canada to guard the representatives of 45 countries attending the meeting, including British Prime Minister Margaret Thatcher and the leaders of several African and Caribbean countries. But Gandhi is the government leader about whom they are most worried.

Canadian and Indian officials said they believe that Vancouver is a center of fund-raising and intrigue for Sikhs fighting for the creation of a separate Sikh homeland in India that they would call Khalistan.

"They can have a Khalistan in Canada," Gandhi chuckled to reporters at a reception, taking a gentle swipe at both the dissidents here and his Canadian protectors, "but not in India."

Canada's handling of radical Sikh groups has been a source of tension and conflict between the two countries with the Indians expressing concern about what they call Canada's laxity in halting violence thought to have been spawned here.

There are more than 200,000 Sikhs in Canada, nearly half of them here. They are scattered through the professions, and many are successful businessmen. Officials in Ottawa say they are a significant political bloc in about one in 10 Canadian parliamentary districts.

Canadian Prime Minister Brian Mulroney ousted the head of the Canadian Security Intelligence Service last month after the agency acknowledged that illegal wiretaps had been used to obtain evidence against Sikhs accused of shooting and wounding a visiting Indian state Cabinet minister last year. The judge set aside convictions, postponing further review until next year.

The disclosures fueled charges by opposition critics in the House of Commons that the security agency and the Royal Canadian Mounted Police had repeatedly bungled investigations of Sikh activity in Canada, including the probe of the crash of the Air India flight that departed from Canada two years ago and exploded over the Irish Sea, killing all 329 aboard.

Canadian prosecutors here also have attempted repeatedly and, thus far unsuccessfully, to convict a radical Sikh leader in Vancouver who is charged with planning bombings of the Indian Parliament and other government buildings. A trial judge threw the case out, ruling that wiretap information could not be used because the government refused to disclose details about an informer who had assisted the probe.

The Sikh leader, Talwinder Singh Parmar, said in an interview with the Toronto Globe and Mail that he is also the target of the Mounties' probe of the Air India crash. He called their purported suspicion a "fiction of the police imagination."

Parmar, a former logger, belongs to a group of Sikhs here, many of them young men in their 20s, who became radicalized after Indian troops stormed the Golden Temple at Amritsar, the Sikhs' holiest shrine, in June 1984.