At least 50 people were injured Friday when severe turbulence hit a Northwest Airlines flight from Manila to Tokyo.

A spokesman for Northwest Airlines in Tokyo said four passengers were hospitalized, including one person with a serious neck injury. Forty-six others, including seven crew members, received light injuries, he said.

A total of 408 passengers and 14 flight crew were aboard the Boeing 747-400 plane, according to the spokesman.

The turbulence hit while the plane was circling east off the coast of Tokyo. The plane suddenly descended about half an hour before landing, throwing passengers without seatbelts from their seats, said the spokesman.

The "fasten seatbelt" sign was on at the time, he added.

Japanese TV networks showed passengers holding their heads and necks as they left the plane.

Fire trucks and ambulances were on the tarmac when the plane landed. One of the passengers was taken away in a wheelchair and another was taken away by stretcher. Some were later wheeled into hospitals on gurneys.

''It really happened so fast and it was over,'' Vincent Salazar, a 55-year-old American passenger aboard the plane, told Kyodo News. ''We were flying and then the seatbelt sign came on and when we were trying to get back to the seats, the plane dropped and so some people went up and hit the top [ceiling]''

''I actually was pretty calm because I've flown a lot, but the lady next to me was screaming... And unfortunately some people weren't in their seatbelts. They were just flying.''

Most of the passengers were believed to have been hurt by hitting their heads or twisting their necks.

A low-pressure system off eastern Japan on Friday morning created strong winds and large thunderclouds that can cause air turbulence.

With files from the Associated Press