Relief workers struggled today to cremate or bury hundreds of victims of yesterday's poisonous gas disaster in an effort to prevent a cholera epidemic. The official toll climbed to 546 and the number of injured to more than 20,000, making it one of the worst industrial accidents on record.

Medical authorities said that the official death toll from the accidental leak of a potent chemical cloud from a pesticide plant of Union Carbide of India does not include up to 300 bodies that have not been registered.

Narendra Bhandari, superintendent of the Hamidia Hospital here, said he expected the number of dead to reach about 1,000 in the next two days. The United News of India said its investigations indicated 1,200 had died.

Prime Minister Rajiv Gandhi, interrupting a campaign trip to visit the stricken area, where people are still dying of the aftereffects, said he had ordered an investigation to find out whether there are other factories in India that might pose a threat to nearby populations.

He said his government would review past policies of allowing such facilities to be situated in densely populated areas. "It is something we have been thinking of even before this tragedy occurred," he told reporters. "This was planning in an uncontrolled manner."

A large pasture behind a colony of slum hovels opposite the plant of the subsidiary of the American Union Carbide conglomerate flickered with mass cremation pyres today, and more than 100 bodies were laid out in rows waiting to be burned as trucks continued to arrive with bodies of more victims to be unloaded by volunteers wearing face masks.

Many of the victims died trying to flee their scrap-wood shanties at the edge of the sprawling Union Carbide facility when a cloud of deadly methyl isocyanate drifted over the area last night following a leak from an underground tank, apparently caused by a faulty valve.

The fumes were carried by a brisk wind more than two miles southeast of the plant, but a depression and the cold air kept much of the gas over Jai Prakash Nagar, a government-sponsored resettlement tract occupied by several thousand poor families.

Some of the gas was carried over a section of Bhopal, threatening many of the 700,000 residents of this capital of the central Indian state of Madhya Pradesh.

The fields around the slum presented a scene of chemical devastation today, their crops of turnips and other vegetables withered and covered with a fine, white film. Dead and grotesquely bloated water buffaloes lay in the fields and atop one another in pens. Pools of water were discolored from the drifting poison cloud.

All 16 burning platforms at the permanent Hindu cremation site at Chola Vishram were smoldering with pyres, forcing relief workers to build temporary mass pyres that turned the grazing field into a furnace as hand carts piled high with wood and trucks stacked with bodies arrived in an almost continuous stream.

Most of the victims' faces bore numbers marked with indelible ink for identification and were partially covered with torn bedsheets as the bodies lay in long rows awaiting the building of new pyres.

Daya Ram, 29, a steel welder who lives a half-mile away, walked alone down a narrow road to a makeshift cemetery, carrying a tiny figure covered with a red dish towel. Ram said it was his year-old son who, according to Hindu custom, was young enough to be buried instead of cremated.

Ram said his family had been asleep when the gas cloud struck, and that he ran, choking and gasping for breath, to take the baby to the hospital, but the child died today.

Ram said some people tried to run away from the choking cloud of gas, but stumbled and fell dead to the ground just outside their homes.

"We never expected this," he said, although other area residents said they long ago had expressed their fears about living so close to the chemical plant.

When asked if he would move away from the plant, Ram replied, "We can't go anywhere. How can we afford to move?"

Some residents of Jai Prakash Nagar said they had heard a siren at about 1:30 at the plant, owned jointly by India and Union Carbide. Others, however, said they had heard no siren and said many of the victims died in their sleep.

"It was smoky and misty. We could barely see the road, and the siren was sounding," said Babu Khan, whose son died and whose wife stood next to him holding a handkerchief over eyes swollen and inflamed by the toxic mist.

Khan said it was not unusual to hear the factory siren, which often sounded because of small fires or false alarms.

Vijay Awasthy, spokesman for the Union Carbide plant, said none of the 100 workers on the overnight shift had been injured by the leak because, he said, "everybody has had safety training and knows how to escape."

He said the safety measures included windsocks to indicate the direction of the winds and show employes where to run to avoid contamination, but he said no such indicators had been installed in the adjacent residential areas.

Awasthy blamed the leak on "too much pressure building up" in the underground tank that holds methyl isocyanate, used to manufacture Sevin and Temik, pesticides commonly used here and abroad.

He said normally accepted procedures had been adopted to prevent such a leak but that a faulty valve unexpectedly released the toxic fumes. The valve was open about 40 minutes before workers were able to close it, officials said.

"There was no way it could be anticipated," he said.

Hari Sharma, an electrical engineer brought by the Indian government to seal off the defective tank, said in an interview that the reason for the high toll in Jai Prakash Nagar was that there was no adequate system for warning the residents when the high-pressure gas shot into the air and was carried toward the slum.

At the Hamidia Hospital, corridors were filled with victims, and the entire parking lot had been converted into a treatment area. Dr. Pradip Chourey said that yesterday the victims showed classic signs of acute toxic poisoning, in which the gas replaces oxygen in the blood and causes pulmonary edema, with patients frothing at the mouth and drowning from liquid in their lungs.

Today, he said, the symptoms were more long-range -- such as blindness from ulcerated eyes.

Hospital superintendent Bhandari said most of the fatalities were people too young or too old to run away from the contaminated area. He said he expected more deaths in the coming days, because the first 72 hours after exposure to methyl isocyanate are critical.

Squatting on the hospital floor, Manorama said the fumes hit her around 1 a.m. while she was in bed with her two children. "It was as if someone threw red chillies at my face," she said. Manorama, her husband and the children ran out of their house in a slum beside the factory and collapsed on the road. Police brought her to the hospital at about 7 a.m., a nurse said.

Nearly 500 other victims lay moaning on the hospital floor, shivering in the morning cold. Rahis Bano, 20, wept for a young son she left behind when escaping the fumes.

"I could carry only one child and now I don't know how to find my other son," she said. "I awoke when I found it difficult to breathe. All around me my neighbors were shouting, and then a wave of the gas hit me." She said she fell down vomiting. Her two sons rolled in agony beside her. She took the nearest child and ran outside before collapsing on the road, where rescue workers found her, she told Reuter.

Madhya Pradesh's chief minister, Arun Singh, said his government would demand compensation from Union Carbide for the victims.

[Walter Goetz, Union Carbide's director of communications, said in Danbury, Conn., according to Reuter: "We are ready to offer aid, financial and otherwise, but compensation is another matter altogether. One is voluntary, the other has to be adjudicated. This is something that would have to be discussed with the Indian authorities and our insurance people."]

The disaster was one of the worst industrial accidents on record. In 1921, a chemical plant explosion in Oppau, Germany, killed 561 people. In 1948, freak weather trapped industrial fumes above Donora, Pa., killing 22 people and making 5,910 ill. A factory explosion in Seveso, Italy, in 1976 released dioxin gas that caused skin ailments but no deaths. In 1979, according to western press reports, hundreds of people died of anthrax in an accident at a Soviet biological warfare plant in Siberia, but the Soviets have said only that there were "a number of victims" in an outbreak blamed on tainted meat.