ABOARD THE VIKRAM, Palk Strait (AP) _ The climax came as dusk fell on the Bay of Bengal, obscuring the Sri Lankan gunboats and the Indian fishing vessels that had faced them for four tense hours in an effort to deliver aid to Sri Lankan Tamils.

''You cannot enter Sri Lankan territorial waters. Turn back,'' the Sri Lankan naval ship radioed.

''Please reconsider your decision. Think of the plight of our small boats on the high sea,'' replied the Vikram, an Indian Coast Guard vessel escorting the 19 fishing boats loaded with Indian government aid.

But the Sri Lankan navy would not relent.

''You turn back or we would be forced to implement our decision,'' said Sri Lankan Cmdr. Mohan Samar Shekhara.

A chill ran through the 124 journalists and Indian Red Cross officials on the Vikram. The crews of the Sri Lankan gunboats tensed. The Indian fishermen switched off their lights.

For a long minute, the only sound was the hum of the engines and the slap of waves.

At 8:30 Wednesday night, the Indian ships turned back, ending the face-off.

India had announced Tuesday that it was sending food, medicine and fuel to civilians besieged in a Sri Lankan army offensive against Tamil rebels. Sri Lanka replied angrily that it did not need the aid and any attempt to deliver it would violate its sovereignty.

The Tamils, most of whom are Hindus, make up 18 percent of Sri Lanka's 16 million people and claim they are discriminated against by the predominantly Buddhist Sinhalese majority. About 50 million Tamils live in southern India, across the Palk Strait from Sri Lanka. They have pressured Indian Prime Minister Rajiv Gandhi to try to end the Sri Lankan strife.

''India tried to do something on humanitarian grounds and if the whole world thinks it was wrong, then something is wrong with the world,'' said Dev Kumar Maitra, a senior Indian official on the Vikram. ''We only wanted to help.''

Maitra tried for four hours to obtain passage for the boats to Jaffna, the largest city on Sri Lanka's Tamil-dominated northern peninsula.

''I had a job to do and I did it to the best of my ability,'' Maitra said as he led a seasick, sleepy group of journalists, Indian Red Cross officials and physicians off the ship this morning in the Indian port town of Rameswaram.

Gandhi had dozens of Indian and foreign journalists flown to the small southern port on Monday to accompany the relief fleet, without waiting for word that Sri Lanka would allow it to land.

The fishing boat fleet, rented by the Indian government, set out Wednesday afternoon with 38 tons of supplies.

As the Vikram was about to enter Sri Lankan waters, a Sri Lankan naval officer commanded, ''Stop there and identify yourself.''

The Indian ship halted and anchored off Kachchativu, a tiny island.

Maitra was asked to board the Sri Lankan ship. He said he would only do si if journalists were allowed to join him - a request denied by the Sri Lankans. The two sides then negotiated over the ships' radios or by shouting across the water.

After word of the Indian retreat reached Sri Lanka's capital of Colombo, the country's national security minister, Lalith Athulathmudali, said, ''Good sense has prevailed. We decided to stand firm but be polite.''

India's Foreign Ministry said in a statement that the government ''strongly condemns the action by Sri Lankan gunboats in forcibly preventing'' passage.

The fishermen were paid the equivalent of about $192 each to make the journey, but several said they had personal reasons for undertaking it. They said 39 fishermen have been killed by Sri Lankan naval gunfire since Tamil rebels in Sri Lanka started a guerrilla war for independence four years ago.

Sri Lanka says it fires on fishing boats because they are used to ferry arms from Tamil rebel bases in southern India.