A gaping brown abyss more than a mile long and 300 feet deep opens here at the foot of the 450-year-old church of the Virgin Mary, the only surviving monument of a town swallowed by its own coal pit.

Yet it is not the razing of the old city for strip mining that most troubles the 60,000 people of Most, who now live in huge concrete apartment blocks nearby. Rather, it is the poisonous haze of sulfur dioxide that clings to the place almost every day, destroying trees, aggravating lung ailments, and assaulting every street with its powerful, rancid smell.

"You should have been here yesterday; it was so bad that everybody had a headache," one local business manager said with a sigh on a recent morning. "The worst days, all the doctors are put on alert. The schools are notified, and children aren't allowed to go outside or walk home."

Air pollution produced by coal-burning power plants has grown so bad in this northern Bohemian region that everyone who works here for 10 years is paid a special indemnity by the government. Schoolchildren are sent out of the area for at least three weeks a year to breathe fresh air. Seeds and sprouts of deciduous trees are imported from abroad to replace the swaths of dead pine forest.

Yet Czechoslovakia, like its communist neighbors, has only begun to address the environmental destruction caused by its thirst for cheap energy and heavy industry. Now, pollution is emerging as one of the most important economic and social issues facing communist leaders.

The northeastern Czech lands form part of a vast area of Central Europe, where pollution has become a major threat to cities, woodlands and human health. To the north, wide tracts of countryside in East Germany are affected by acid rain and other pollution from the unfiltered coal-burning power plants of Leipzig and Halle.

To the east, Poland's heavily industrialized Silesia region has grown so polluted that rates of respiratory ailments and cancer range 30 to 45 percent above the national average. Monuments in the historic city of Krakow are blackened and slowly crumbling from erosion by airborne chemicals.

Increasingly, Communist authorities in Czechoslovakia and East Germany have come under pressure from western neighbors to curb emissions. West German officials such as Franz Josef Strauss, governor of Bavaria, have even donated test filtration equipment for coal-burning plants in Bohemia.

Pollution also threatens to become a source of domestic political unrest. Even the tightly controlled Czechoslovak press has published critical accounts of the destruction of the country's forests. In Poland, public concern has become so great that a political scientist recently predicted that the next political crisis would be a rebellion against pollution.

In response, Eastern European governments recently have begun to take their first major steps on environmental problems. Western businessmen reported unprecedented interest by Communist officials in antipollution equipment at the recent Leipzig trade fair. Poland and Czechoslovakia have pledged to double spending on ecological problems in the next five years, but officials and diplomats say solutions to the pollution problem in Eastern Europe may take decades.

"If one looks at the region, the outlook for real improvement is not good," a western diplomat in Prague said. "A lot of the problem comes from coal, and not for a very long time will these countries be able to dispense with coal-fueled industrial plants."

With its extensive but antiquated industrial base, Czechoslovakia faces perhaps the most serious problems of adjustment. Most of its power comes from burning sulfurous coal, and as supplies of oil from the Soviet Union have been cut back since 1980, dependence on this fuel has steadily increased.

In Most, the thirst for new coal supplies caused authorities to tear down most of the old city center in the 1970s for a vast strip mine for lignite. It now supplies huge chemical works and power plants that until this year had no filtration systems for their emissions.

While officials say power plants and industries in Bohemia are slowly being converted from coal to gas or supplied with antipollution systems, the situation in Prague is close to unmanageable. About 200,000 chimneys in the historic city's older districts are connected to coal-burning furnaces.

In the center of Prague, where 50,000 residents are joined by 250,000 workers each day, levels of pollutants like sulfur dioxide can reach three times the acceptable health standard, according to Jan Skoda, head of the city's Commission of the Council of the Environment. "These conditions have been developing for decades, or since the last century, in a very unfavorable way," he said.

Skoda said almost all trees and buildings in the central city had been damaged in recent years by air pollution, and two-thirds of the city's sewage was being dumped with little or no treatment into the Vltava (Moldau) River, which winds through the center of town. Nitrate pollution of drinking water is so severe that women with newborn children get priority access to mineral water to prevent infant illnesses.

A report prepared by the city several years ago but not released publicly said 65 percent of Prague's springs were contaminated with nitrates by 1981, and air pollution was shortening Prague's daylight at the rate of one hour every 10 years. It concluded that the environmental situation "acutely threatens the health and life of citizens."

Other reports have also been alarming. A 1983 study by the Academy of Sciences that was leaked to the dissident group Charter 77 said 2 million acres of Czech forests had been damaged by acid rain and that "up to 30 percent of the fish, bird and mammal life is threatened."

Officials say such threats are being countered with major plans for investment in coming years. Vaclav Vertelar, first vice president of the State Planning Commission, said coal mining and consumption would be cut back 10 percent in the next five-year plan, and about $1.5 billion would be invested in new treatment stations for air and water.

Officials note that Czechoslovakia recently agreed with other European countries to reduce its sulfur dioxide emissions by 30 percent over 1980 levels by 1994.

Nevertheless, officials say, only after the turn of the century do they expect to reduce air and water pollution in Prague and north Bohemia to acceptable health levels.

"We are not satisfied," Skoda said. "We know it's not healthy for people, but the solutions are very complex."