More than 800,000 Southeast Asians have fled their homelands for an uncertain future in the past year and a half, making the region's movement of homeless people one of the greatest migrations in recent times.

Their wanderings in search of a safe place to live represent the lingering fallout of conflicts that have changed the face of Southeast Asia since World War II and come on top of already considerable human migration during years of warfare.

Countless numbers in the recently swollen tide have died before reaching a haven, which in itself may be no more than a disease-ridden refugee camp. What's more, the arrival of survivors in other countries of the region is becoming increasingly unwelcome as fragile social and economic systems become strained to the breaking point.

Spurred by the saga of the Vietnamese "boat people," whose plight is symbolized by pictures in newspapers of pleading mothers and tearful children pushed back into a raging monsoon surf, representatives of about 40 countries are gathering in Geneva this week to try to resettle the many thousands stranded in squalid camps.

Aside from the United States, France, Canada and Australia, few countries have made more than a token effort to take those remaining in camps in Thailand, Malaysia and other non-communist Southeast Asian countries -- the primary focus of the Geneva talks.Beyond these, however, are hundreds of thousands of others who have fled from Cambodia into Vietnam and from Vietnam into China.

Those who fled by sea have captured estimates they represent only a fraction of the total.

No one knows with certainty the numbers of those who have fled from Vietnam, Cambodia, Laos and Burma, but the known number of Vietnamese boat people is about 85,000 -- about a tenth of the estimated total given by State Department and other experts.

The greatest unknown is the number who have perished at sea. Malaysian authorities put the number at 400 in the past several weeks in their coastal waters alone. It is the monsoon season in the South China Sea and the raging waves are merciless to the small coastal craft in which most of the Vietnamese flee.

Far more numerous than the boat people are the ethnic Chinese who have fled from Vietnam north across the tension-ridden border into China. Experts who follow Chinese affairs put this number at about 170,000 as of late summer and believe others are certain to have slipped across since then. Some are even showing up among Chinese trying to sneak from the mainland to Hong Kong.

Cambodians who have fled the turmoil in their country are believed to number at least 180,000 -- about 30,000 into Thailand and 150,000 into Vietnam.

Thailand also has received as estimated 125,000 people from Laos, mostly Meo hill tribesmen who were trained to fight by the CIA and now face extinction under a concerted drive by the Pathet Lao government apparently aided by a sizable Vietnamese force,

Added to these -- most of whom are part of a political and social realignment flowing from the Indochina wars -- is the bizarre tale of almost 200,000 Moslems who fled last May from Burma to Bangladesh and only just now are beginning to return to their villages.

These fugures are believed by experts to represent people who have fled since the summer of 1977. If the number who left their countries immediately after the fighting in 1975 is added, the tota is certain to be far over a million people.

This total, while perhaps not as great as the tumultuous migrations that occurred when the Indian subcontinent was divided or the ethnic Germans were expelled from Eastern Europe at the end of World War II, ranks among one of the greatest movements of peoples in recent times.

For the countries receiving these vast numbers of people, the impact has been traumatic, aggravating economic uncertainly and historic racial tensions alike.

Malaysian officials, during a recent visit to Washington to press their case for faster action to resettle refugees, said their local facilities are strained to the breaking point, and that tensions between the Vietnamese and the local populace on their northeastern coast are rising rapidly.

According to the Malaysians, families are being uprooted to make room for receiving and settlement camps, hospital facilities are overcrowded with refugees, food supplies are drying up and Malaysians are being mobilized to handle logistics for the refugees.

While the Malaysians vociferously deny there is concern over the fact that many of the boat people are ethnic Chinese, outside experts say that in private there have been great expressions of concern that the country's delicate balance between Indians, Chinese and Malays could be upset by the new arrivals.

Similar racial antagonisms are believed at play in Thailand, where experts point to longstanding tensions among Thais, Vietnamese and Laotians.

"It gets to the heart of ethnic feelings and you don't change history overnight," one expert observed.

The Thais are particularly worried about the hill tribesmen from Laos. While a few with skills or with a profession have been taken by the United States or France for resettlement, most are simple farmers or possess very limited skills. The Thais reportedly have expressed fears that they may become permanent residents in their camps.

Complicating the problems for receiving countries and those who would take refugees for resettlement is the unexpected upsurge in the numbers of people leaving Vietnam.

The first rush that left immediately after the end of fighting in 1975 were primarily people closely linked to the defeated South Vietnamese government or the United States and who therefore feared for their lives under the new Communist government.

Throughout 1976 and 1977 a reative trickle of people came from Vietnam as the realities of life under the new government became apparent. Last March, however, the new government outlined a sharply changed economic system, eliminating most of the remaining small businesses.

This drastically affected the ethnic Chinese population in Vietnam -- already on edge as a result of tensions between Vietnam and China -- and the exodus began. When it became clear that southern Vietnam was suffering a major shortfall of food, the outflow became a flood this past summer, over the border into China and by boat.

This new movement of peopel added an unexpected quarter of a million refugees to the already huge total from Cambodia and the swelling tide from Laos.

Meanwhile, on the Burma-Bangladesh border, a diplomatic agreement between Dacca and Rangoon has broken an impasse over the fate of up to 200,000 Moslems who fled from predominantly Buddhist Burma into Moslem Bangladesh beginning in May and hve been housed under miserable conditions since.

The reasons for the outflow remain unclear, but appear to be rooted in separatist sentiments in the region from which the Moslems fled and fears among some of them that Burmese authorities were launching a vendetta against the religious minority.

After initially refusing apeals from Bangladesh to take the people back, Burmese authorities have agreed to repatriate some 130,000 who have proven Burmese papers.

The Moslems, however, have been reluctant to return, fearing that their houses have been occupied and their meager lands confiscated. In attempts to win back their confidence, village elders are being taken back to show them that conditions are safe, in hopes that they can persuade the others to return.

While some refugees have been resettled, feelings have been mounting in Southeast Asia that Western countries, and particularly the United States, have been doing far too little.

Malaysian Home Minister Muhammad Ghazali Shafie recently told officers at the Malaysian equivalent to the U.S. National War College that "countries who contributed toward the creation of this problem just cannot turn their heads and wish it away."

"They are not fulfilling their obligation in the context of human rights simply by offering money to the U.N. high commissioner on refugees and dictating from a distance what Malaysia should or should not do," Ghazali said, according to a transcript of his remarks. "Conscience money is not the answer."