A Northwest Airlines jetliner with at least 149 people aboard plunged in flames onto Interstate 94 just after takeoff from Detroit Metropolitan Airport last night, apparently killing all on board and injuring an undetermined number of motorists on the freeway.

The plane, a McDonnell Douglas MD80 en route to Phoenix and John Wayne Airport in Orange County, Calif., burst into flames after climbing only 300 feet, according to witnesses and aviation officials. A Federal Aviation Administration spokesman said the fire erupted on the jetliner's left side as it departed at 8:45 p.m. (EST).

If all aboard were killed, as is likely, it would be the third worst aviation disaster in U.S. history, behind the May 25, 1979, crash of an American Airlines DC10 that killed 273 in Chicago and the July 9, 1982, crash of a Pan American Boeing 727 at Kenner, La., near New Orleans, that killed 154.

"It's just an unbelievable mess out there," said Officer Fred Berry, a Wayne County sheriff's officer stationed at the airport. "There are bodies on the road and you cannot tell who is who. I have never seen anything like this. It's a complete disaster."

The jet crashed into the intersection of I-94 and Middlebelt Road, just beyond the airport perimeter. According to officials on the scene, the plane slid almost half a mile after it hit the ground.

There was some confusion as to how many people were on the flight. An FAA spokesman said there were five crew members and 144 passengers. But a spokesman for Northwest Airlines said 153 people were aboard.

"It is just too soon to say for sure what happened to the flight and why it crashed," said Bob Buckhorn, a spokesman for the FAA. "We are investigating now and we will have more answers tomorrow."

A team of investigators from the National Transportation Safety Board led by board member John Lauber left National Airport for Detroit at midnight to begin an investigation.

The crash of Northwest Flight 255 was the first major plane crash in the United States this year.

Police at Detroit Metropolitan Airport said that state, federal and local police officers and voluntary agencies were attempting to transport bodies to five area hospitals.

"But there is nothing we can really do to help them now," said Berry.

There were initial reports of injuries to at least five drivers passing by the airport, and there were unconfirmed reports of people killed on the ground.

One of those injured on the ground was a 4-year-old girl, who had burns over 20 to 30 percent of her body. She was originally taken to a Wayne County hospital but was airlifted to the University of Michigan hospital, where she was in critical condition.

Spokeswoman Catherine Cureton said the hospital's Survival Flight Helicopter happened to be in the vicinity when the flight went down. "The first person they found was a little girl in a car near an engine of the plane," she said.

The Associated Press reported that an airport security guard, Gordon Atkins, on his way to work at the time of the crash, also wound up hospitalized, though with less serious injuries. He said he unsuccessfully attempted to help ground victims who he assumed were dead.

Atkins, 26, was treated at a hospital and released. He said he had tried to rescue a man from a vehicle that was hit by flying debris on Middlebelt Road near Interstate 94.

Atkins said he was driving south on Middlebelt when he saw the vehicle in front of him, a Ford Bronco, burst into flames and start rolling down the street.

"I saw a guy burning in a Bronco and threw my jacket on top of him. My jacket burned up. I tried to do the best I could, but the heat was so hot I couldn't stay.

"The guy was burning and I guess he died."

Atkins' airport security guard uniform shirt was bloodstained. He said after trying to rescue the man in the burning vehicle, "I saw a guy lying by the {I-94} viaduct. I tried dragging him out of there. That's where I got all this blood, but I couldn't drag him. It was just too hot."

He said that when "the plane blew up, a piece of it hit me on the hand. I was right under it."

He described the scene: "First thing I heard was a loud noise like the whole world was coming down on top of me -- a loud screaming noise . . . fire came rolling down the grass -- I guess it was the fuel, I don't know."

The airport, which was closed immediately after the crash, reopened within a half hour, according to Wayne County Executive Edward McNamara.

Witnesses say the plane was already burning when it went down.

"The plane was on fire and the wings were swinging back and forth," said Russell Swett, a restaurant patron who saw the crash.

Karen Pointon, 22, of Romulus, Mich., said she and her fiance watched the plane crash about 1,500 yards from the back yard of her home just outside the airport.

"All you could see was the street glitter with flames," she said. "We saw a big ball of yellow flames roll down the street."

The plane also hit the airport Avis Rent-A-Car building and exploded, according to police officers.

The accident appears to the first in the United States involving the MD80, the updated variation of the DC9 that was introduced in the early 1980s.

A spokesman for United Technologies, whose Pratt & Whitney Division built the engines for the plane, said last night that the fleet was in excellent shape and that he had no reason to believe the accident would suggest that the nation's MD80s should be examined for flaws.

Donald Hanson, director of public relations for McDonnell Douglas, said the MD80 -- a twin-engine plane built for short- to medium-length flights -- has "had a very good record all along. There have been no serious mishaps with the MD80."

"There have been cases of engine problems where the blade has been thrown from the engine, but no injuries were caused," he said.

The passenger list was not immediately available last night.

Friends and family of people on flights from the Detroit airport rushed to the scene as soon as they heard of the crash. Police, firefighters and volunteers immediately closed the area around the airport to vehicles, tying up traffic and sending many people to nearby hotels to telephone airlines.

Officials on the scene said that they had recovered the plane's flight recorders, the "black boxes" that record pilot conversations and airplane performance characteristics.

The investigators from the NTSB will use the boxes to attempt to discover whether the pilot was aware of problems with the aircraft immediately, and what caused the fire.

The last fatal crash at Metropolitan Airport occurred March 4, when a Northwest Airlink commuter plane lost power while landing and then crashed, burned and slid along a runway. Northwest Airlink is affiliated with Northwest Airlines, but is not operated by the company.

The pilot, copilot and seven passengers died and 13 others were injured in the crash, which at the time was the worst in Metropolitan Airport's 30-year history.

Northwest has been plagued with labor problems since it merged with Republic Airlines last October. The problems included reports of vandalism, although there were no indications that vandalism contributed to the crash.

The FBI said in January that it was investigating reports of tampering with Northwest planes at Minneapolis-St. Paul. And the Detroit Free Press reported in July that the airline had hired security guards to prevent tampering with buildings and equipment at the Detroit airport.

The Associated Press quoted Northwest as saying that flights from Detroit were being delayed by a slowdown by members of the International Association of Machinists, although union officials denied there was a slowdown.

The U.S. Department of Transportation reported in July that Northwest ranked second behind Continental Airlines in the number of customer complaints during the first six months of 1987 with 8.76 complaints per 100,000 passengers carried. Northwest also ranked second behind Continental for complaints received in July.

United Press International reported that some witnesses, including an air traffic controller, said the plane may have been on fire before the crash, prompting an FBI investigation.

"We are investigating the possibility that there was a bomb on board," FBI special agent John Anthony told UPI. "It is only speculation, but we are investigating it. There were several eyewitness accounts saying the plane was on fire before it crashed."