In the fullest official account so far of what happened at Chernobyl on the night of April 25-26, the Communist Party daily Pravda today described an explosion ripping through the nuclear reactor's housing and a roaring fire followed by discharge of radioactivity into the atmosphere.

A summary of the Pravda account carried by the Tass news agency concluded that the situation around Chernobyl, 60 miles north of Kiev, "remains complicated," although "under control."

In a dispatch compiled by correspondents from the area, Pravda gave not only the first vivid details of the accident itself, but described how the area around the plant was evacuated in the space of four hours despite panic among some "individuals."

Now, Pravda said, the city of Pripyat, which had a population of 25,000, is a virtual ghost town. "Only a specialized radiation-monitoring motor vehicle appears on the streeets from time to time," it said.

Last night, a Soviet government statement carried by Tass said work had begun to bank up the Pripyat River near the damaged reactor to prevent its possible contamination from radiation. It said people evacuated from an 18-mile zone around the power station would be given temporary jobs elsewhere while a cleanup of the site continues. It made the first mention of effects felt in Byelorussia, which begins eight miles from Pripyat.

Pravda said "additional measures" have been taken to deal with the accident, and the environment is being monitored in "Kiev, Chernigov and other major cities and small settlements." The radius of the testing program was not given, although the mention of other cities suggests a broad sweep.

Until now, accounts of the accident have been pieced together from spare official statements, details provided by Soviet officials abroad and unofficial reports from the area.

The Pravda account indicates that the fire following the explosion in the 100-foot-high housing melted the asphalt roof -- which was the means of access to the blaze for firemen who reached the scene. "The firemen were fighting the blaze . . . . Their boots stuck in bitumen that melted because of high temperature; soot and smoke made it difficult to breathe but the brave, bold men kept fighting the blaze courageously," said Pravda.

The newspaper quoted specialists as saying later that the firefighters' efforts "limited the scale of the accident to a considerable extent."

Still, radioactivity was "partially discharged upwards," Pravda said, and a fire then "started inside" -- presumably within the reactor itself. The paper noted that since neither water nor chemicals could be used to put out the blaze, "a complicated and extremely difficult situation took shape."

Pravda said the evacuation of the area took four hours. "Although there were some panic-stricken individuals . . . , the mishap rallied the people so closely together that they quickly restored order themselves."

Soviet officials abroad have put the figure of people evacuated at 49,000 from four settlements around the power station.

Pravda reported that people were taken to neighboring districts, where the children have been assigned to schools and where adults are "helping local residents in their work." From the description, it would appear that people are not expected to return to their homes soon.

The paper gave special praise to volunteer drivers from Kiev who helped evacuate the stricken population "although it was a Saturday." Doctors also reported to city hospitals to offer help, it said.

The detailed account and last night's government statement, which was read on the main evening news program, are part of a series of daily reports which bit by bit have suggested the scope and possible long-term consequences of the accident at the Chernobyl plant.

Tonight's government statement said that radiation levels in the Ukraine and Byelorussia were "stabilizing with a tendency towards its improvement." As has been true since Monday, when the accident was first announced, no official figures on radiation levels were provided.

Last night's statement was the first to mention Byelorussia, a Soviet republic, in connection with the accident. Western specialists said it may have the highest radiation levels because it lay in the path of the radioactivity released in the accident.

The statement did not elaborate on what measures were being taken to protect the Pripyat River, where the power plant is located. The Pripyat feeds into the Dnieper, a major river and source of water for Kiev, with a population of 2.4 million.

Nor did it indicate whether people in the area had been cautioned against drinking, fishing or bathing in the Pripyat River.

Another item on the television news tonight was the arrival in Moscow of the head of the International Atomic Energy Agency, Hans Blix. He came at the invitation of the Soviets, accompanied by deputies Leonard Konstantinov of the Soviet Union and Morris Rosen of the United States.

The main Soviet evening news program last night devoted almost 13 minutes to Chernobyl and its region, most of it emphasizing that life was continuing as normal. There was also a commentary again accusing the western press of fomenting hysteria over the accident.

Interviews in the streets of Kiev, on a farm in Byelorussia and on a state farm in the Ukraine 25 miles north of Kiev showed people back at work after last week's holiday, which began May 1 and lasted through Sunday -- Easter on the Russian Orthodox calendar.

A farmer, in an interview after film footage showed cows grazing in fields, said the only unusual occurrence after the accident was the arrival of equipment to measure the air for radiation. So far, he said, the measurements had shown no "elevations above normal standards."

In Kiev, one man said the evacuations from the area around Pripyat had been orderly, "done in the best way."

"There is no panic," said another man stopped on the street. "The mood, I would say, is quiet and normal," said a young man.

The people interviewed said the greatest excitement in Kiev is an international bicycle race nearby today. Some West European teams have said they would not take part because of fears of radiation. The race goes through parts of Poland and East Germany and finishes in Prague on May 22.

In interviews given in West Germany to foreign news organizations, Boris Yeltsin, head of the Moscow city Communist Party, has said that 49,000 people were evacuated, and that 25 were in serious condition.

Yesterday, Yeltsin was reported as saying in Hamburg that radiation is continuing to seep from the reactor, but that it has almost been plugged. On Friday, he described how helicopters had dropped lead, sand and the neutron-absorbing element boron on the site to stop the leak.

Last night's statement said, "The emission of radioactive substances continues to decrease" and "the necessary sanitation, hygienic, treatment and preventive measures are being carried out."

Blix, of the IAEA, said he had come to Moscow to continue talks about the accident.