Indian airports and airlines tightened security today amid a rash of bomb scares here and abroad following Sunday's mid-air explosion and crash of an Air India jetliner off the coast of Ireland.

The crash of the Boeing 747, bound from Toronto to Bombay via London with 329 passengers and crew, is suspected to have been caused by a bomb placed on board. A Sikh extremist organizations and a group seeking independence for the northern Indian state of Kashmir have claimed responsibility for the crash. A suitcase bomb that killed two cargo handlers and wounded four other persons at Tokyo's Narita Airport has also been linked to the Air-India crash.

Air-India and a domestic carrier, Indian Airlines, reported receiving a number of telephoned bomb threats from anonymous callers since the crash of the Air-India flight, causing delays on international and internal flights. Airline spokesmen said all domestic flights were being delayed because of more stringent security and baggage checks.

In the Indian city of Indore, a Sikh youth boarding a flight to Bombay with a loaded revolver hidden in a briefcase was arrested last night, police said. The same flight was the target of an anonymous bomb threat, but no explosives were found, authorities said.

There has been no sign so far of any backlash against Sikhs here by the majority Hindu population to retaliate for the suspected bombing of the Air-India flight.

Prime Minister Rajiv Gandhi and his supporters in the ruling Congress (I) Party have appealed for calm. Police armed with bolt-action rifles or long batons -- on alert since a wave of terrorist bombings in the capital in May that police have blamed on Sikhs -- have been patrolling to prevent any repetition of the anti-Sikh violence and rioting that erupted after the assassination by Sikhs of Prime Minister Indira Gandhi last year.

Militant Sikh leaders in India and Britain denied any involvement in the Air-India crash and strongly denounced any act of sabotage that may have been involved in it. Some pointed out that both the pilot and copilot of the plane and as many as 60 of the passengers were Sikhs. However, they could not explain why a Sikh militant group seeking an independent state called Khalistan in the Indian Punjab had claimed responsibility for the crash.

R.K. Thadani, a regional director for Air-India, said that in the last 24 hours two bomb threats against the airline's planes had been received from unknown callers in Bombay and one or two in London. One Air-India flight en route from London to Bombay was delayed by a bomb scare for seven hours in New Delhi last night, airline officials said.

It could not immediately be determined whether any of the hoaxes were connected with the Air-India crash and the Narita Airport blast, or whether they were the work of pranksters.

The bomb scares came as grief-stricken relatives of passengers aboard the crashed Air-India plane besieged the airline's offices here seeking word of efforts to recover the bodies from the Atlantic. Airline officials said 135 had been recovered so far, but strongly discouraged relatives from flying to Ireland to try to indentify them.

Much of the anger generated in the news media here about the crash seemed to be dircted at foreign governments that allegedly failed to take account of terrorist threats.

In an editorial headlined "Murder in the Sky," The Hindustan Times said today that Canada, where the ill-fated flight originated, "and many European countries have been remarkably unreceptive to the Indian government's warning about various anti-India extremist groups operating from foreign territories."

It accused authorities at Montreal Airport of conducting a "callously haphazard security check" that allowed a powerful explosive device to go onto the aircraft despite an alert caused by dogs trained to sniff out explosives. The editorial called for a halt of Air-India flights to Canada.

Air-India officials said today that weekly flights to Canada were being temporarily suspended but denied this was for security reasons.