United Flight 1676, a Boeing 737-700, landed in Billings at 1:23 p.m., completing its flight from Denver with its 114 passengers and crew members.
MTN reported passenger Bill Dahlin said the plane dropped sharply and without warning, apparently surprising not only the passengers but the crew.
"There was a lot of screaming, a lot of hollering," Dahlin said, adding he thought the crew "somewhat panicked."
"I think they were trying to assess things themselves so they really didn't offer any explanation because of what happened so quickly," he said.
He said one woman hit the ceiling so hard she cracked a panel and he heard another woman call out for her baby after possibly having lost control of the child.
United officials said three crew members and a number of passengers were injured, KUSA-TV, Denver, and the Billings Gazette reported.
United spokesman Luke Punzenberger said the airline's "primary focus is assisting out employees and passengers who were injured,and our flight safety team will review what happened," KUSA-TV said.
United spokeswoman Christen David said Flight 1676 was jolted while descending into Billings, the Gazette said.
"The flight encountered severe turbulence upon descent into Billings, Montana," she wrote in a release. "Three crew members and a number of passengers were injured."
Gregor said the flight's captain called a medical emergency as the jetliner approached Billings.
"The plane landed in Billings without incident," Gregor said in an email.
Emergency crews and the airport fire department responded, the Gazette said.