LONDON, NOV. 19 -- Survivors of London's worst subway fire said today that police had allowed new passengers to descend into the smoke-filled tunnels last night even as those below were struggling to get out.

During a brief period while the fire was still smoldering, before it had burst into searing flames, "a number of people were coughing in the fumes and trying to make their way out as quickly as possible" along the escalator where the fire began, said businessman Stephen Holt, 32.

"There were still other people actually coming down into the station at this time."

Fire and police officials played down the reports, noting that what had initially appeared to be a small and isolated fire had burst quickly into an inferno of such intense heat that it cracked the concrete walls and burst ceramic tiles off the ceiling.

Thirty persons were killed and dozens were injured in the disaster, which took place in London's largest and busiest underground station, at King's Cross.

The officials said they still had no firm idea of the cause of the fire, which apparently started on an escalator leading up to the main underground ticket hall just beneath the street.

They discounted early reports that wooden slats on the steps of the decades-old escalator had played a part, or that machinery or cigarette sparks had ignited rubbish in a room beneath the moving stairway.

The room, they said, was clean, and the escalator wood was barely charred. There was some speculation that the fire could have started in electrical insulation, but officials said they would have to sift through massive amounts of debris before they came to any conclusion.

Prime Minister Margaret Thatcher, who visited the site today and descended into the charred wasteland that had been the ticket area, announced a public inquiry into the disaster.

There was mounting insistence by firefighter unions and public pressure groups that safety procedures are not up to standard in the 254-mile underground rail system used by 2.5 million riders daily, and that budget cuts had brought station staffing to dangerously low levels.

In a report two years ago following an earlier fire at the Oxford Circus station in central London, a public interest group had recommended installing ceiling sprinklers and smoke detectors at the underground stations, fire doors to stop the spread of smoke, and clearly marked emergency exits.

Transport officials acknowledged today that none of these measures was implemented. But they noted that other changes, including the banning of all smoking in trains and stations, had been made.

Opposition Labor Party member of Parliament Frank Dobson, who represents the King's Cross area, charged in the House of Commons that the station, with eight platforms and a "rabbit warren" of tunnels, had been cut from 16 employes to 10, with cleaning staff reduced from 14 to two, and that staff emergency training was insufficient.

The installation of automatic ticketing machines in recent years has reduced the staff in many stations, where the only regular personnel on duty in main station areas usually are one person to sell tickets to those without change and one to collect tickets from exiting passengers.

But Tony Ridley, spokesman for London Regional Transport, which operates the underground system, said, "I don't believe that staffing levels have had anything to do with the fire last night. We will, of course, look at procedures . . . {and} staffing numbers . . . . Until we know how the staff acted precisely, we will be totally unable to say whether additional staff would have been of any assistance. I personally doubt it."

Public officials, including Thatcher and Queen Elizabeth II, praised the courage of rescue workers who plunged deep into the searing heat and smoke-filled tunnels, described by one as akin to climbing down a blazing chimney. One firefighter was among the dead, and two were injured.

Assistant Chief Fire Officer Joe Kennedy, who was in charge of the 150-man force that battled the inferno for nearly eight hours before it was completely extinguished, called it "the worst that I've seen in my 26 1/2 years on the force. The conditions were terrible."

The immediate cause of many of the deaths was inhalation of the choking smoke, which survivors likened to that caused by burning rubber, and the intense heat.

Ironically, those who stayed deep in the bowels of the station survived. Since the fire was close to the surface, most of the casualties were those who obeyed the human impulse to get out of the station.

Most of the dead were badly burned, and at least half the bodies remained unidentified tonight.

The fire began shortly before 8 p.m. in the upper levels of King's Cross, which serves five subway lines and connects to a mainline station of British Rail, the nationwide train service.

Passenger Holt, who escaped the blaze, said there was little initial panic as smoke started billowing through the station and tunnels. As he was coming up the escalator, he told the British Press Association, "the smoke was getting much thicker . . . . Halfway up the escalator, you could feel the heat and at the top, near the ticket collection area, there was a lot of heat around and the smoke was becoming much thicker and denser.

"It seemed like burning rubber," he said. "A lot of people were coughing and covering their faces with handkerchiefs or scarfs. I put my jacket around my face to get out as quickly as possible.

"It was clear by this time that there was something seriously wrong.

"There were some elderly people who clearly could not move very quickly," Holt said. "I was running by this time. I got to the bottom of the steps," leading to the street outside, "and here the smoke was extremely thick -- you could see it billowing outside. As I came out into the air coughing, spluttering and choking, I was followed by about six other people.

"After that, no one else came out."