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Two crowded ''sky bridges'' inside the lobby of the Hyatt Regency Hotel here collapsed tonight, killing at least 45 people and injuring about 145, according to the police and rescue workers.

Four and a half hours after the disaster, with the ruins of the walkways remaining to be lifted from the floor, the authorities said the toll could go much higher once the debris was removed with heavy equipment sent to the scene.

''We have found six people still alive who are trapped between the two skywalks,'' said Sgt. Jim Treece, a spokesman for the Police Department. ''We still don't know how many people are trapped, but there were 1,500 people in the lobby when it happened, and God only knows how many are still in there.''

Room Used As a Morgue

Most of the 1,500 had been attending a popular Friday night dance held in the hotel's lobby. Officials used one of the rooms in the luxury hotel, situated just south of the downtown area near the Crown Center development, for a temporary morgue as bodies were being pulled from the tangled wreckage of twisted girders and smashed glass. Police Chief Norman Caron said late last night that there were 45 confirmed deaths and that about 145 had been treated for injuries. He said a city bus had been assigned to transport the more than 80 walking injured to hospitals for examination and treatment.

More than 40 emergency vehicles from throughout the metropolitan area converged on the scene and helicopters were pressed into service to take away some of the injured.

Rescuers dug into the debris, guided by calls for help from trapped victims, to bring out survivors, some of whom were wandering about the hotel lobby in evening clothes, bleeding and dazed.

Water gushed into the lobby from fractured pipes, the ends of which could be seen sticking out of the wall. The hotel's main entrance quickly became flooded, but officials reported that the rescue efforts had been well-organized and were proceeding as fast as possible. 'Watched It Cave In'

A witness said she had heard a loud noise, looked up and saw what she believed to be the third-floor balcony fall on the one crossing at the second-story level.

Another witness, Dorothy Johnson, a nurse who was dining in a restaurant overlooking the lobby, said, ''You couldn't believe it. It just started crashing, caving in. We just sat there and watched it cave in.''

The ''sky bridges'' of the Hyatt, a 43-story hotel that opened just over a year ago, were built of concrete and reinforced with steel. They are stacked one over another at various levels in the soaring lobby of the $50 million hotel.

One of the tallest buildings in Kansas City, the Hyatt Regency opened July 1, 1980. One of its most striking features was the scene of the disaster, a 60-foot high lobby that formed a four-story atrium walled in by glass and criss-crossed by the walkways high above the lobby floor used for a dance.

A hotel spokesman described the structures as ''balconies that span the lobby.'' There were three of them, usually referred to by the hotel as ''sky bridges.''

The police said one of the walkways cracked, fell and knocked down the other crossing the open space underneath. The walkways then tumbled down onto the enclosed courtyard onto the dance floor in the lobby, taking a chunk of the hotel's interior structure with it.

A fireman, Tom Walker, said the third catwalk was ''separating from the structure and there's a good chance it will collapse, too.'' Mr. Walker added that it was ''terribly unsafe in there.''

A staff member at the hotel said the accident occurred about 7:15 P.M. (8:15 P.M. New York time). Many of the victims were apparently attending a weekly dance contest called the ''Tea Dance.'' Natural Gas Leak Reported

According to officials, rescue operations were impeded by leaks of natural gas. Efforts were being made to close the leaks in order to forestall the possibility of an explosion. But a spokesman for the local gas company denied reports that there had been a break in any gas mains.

A spokesman for the Kansas City Fire Department, Harold Knabe, said part of the lobby on the west side of the hotel had also fallen. Cranes and other heavy equipment were being used to remove large chunks of the fallen structure so rescuers could aid trapped victims.

''That whole thing just collapsed,'' Mr. Knabe said. ''We don't know how many people are under it.'' Mr. Knabe estimated that there had been 500 people in the main lobby and on the two walkways when accident occurred.

Altogether about 2,000 people had come to the hotel tonight for the dance, a Big Band event that was just drawing to a close when the collapse occurred.

Mayor Richard Berkley stood in front of the hotel as bulldozers dug into the debris. He called the accident a ''very serious tragedy.''

''The fatalities are here being identified,'' the Mayor said. The lifting of the crosswalks will certainly produce more victims.'' Later, when the death toll had climbed past 40, Mr. Berkley voiced fear that there might have been ''many, many more deaths.'' 'We'll Make Do'

Truman Medical Center, which holds the Jackson County Morgue, was being readied to receive the dead. The morgue has facilities for 12 bodies, but hospital officials feared that as many as 70 might be sent to them.

''What it amounts to is we'll put them on stretchers or the floor,'' said Dan Couch, associate executive director of the medical center. ''We'll make do with what we have.''

Tom Edgerton, an assistant hospital administrator who was at the Hyatt in a second-floor bar, said there was a ''festive'' mood, with people ''having a good time.''

Then, he said, ''There was this huge roar that sounded like something was falling, falling, falling. I ran out to see what had happened. It was billowing dust and people were gaping. They were just in shock,''

Another witness, Sandy Goodrick, reported having walked into the hotel for a drink just five minutes before the collapse. Two of her friends were trapped under the falling debris and she herself suffered cuts and bruises of the face, neck, arms and legs. 'Everything Just Fell Down'

''I was right underneath when it fell,'' she said. ''We just walked in and were by this little couch when this horrible explosion occurred and everything just fell down.

''There were people screaming for help and trapped underneath, but we couldn't do anything to help them. ''There was a man walking out with a woman and her arms and her leg was gone,'' she continued. ''Later he said she died in his arms.'' Ann Dunford of Kansas City was with her husband, Randy, and a friend from Texas when they decided to go on one of the skywalks to watch the dancers below. ''I had one foot on the skywalk, and I don't know if I felt it or heard it give,'' Mrs. Dunford said.