A Brazilian airliner has made a safe forced landing after a passenger had a "psychotic attack," entered the cockpit and assaulted a pilot, crew members and passengers who tried to subdue him.

The TAM Airlines jet was en route from Montevideo, Uruguay, to Sao Paulo when a man who some passengers said was wearing a TAM identification badge got into the cockpit because the door was open.

Shortly after, the plane dived to the right, causing passengers to scream and the pilot to yell for help over the speaker system.

"All of a sudden shouts were heard from inside the cabin, and two stewardesses came out asking for help and grabbing a young man by his feet," said one unidentified passenger.

"The people quickly jumped on top of him, the guy fought back furiously, biting and hurting a half dozen people.

"In a kind of collective action they were able to immobilise him, tying him to a seat."

Uruguay's Industry Minister Roberto Kreimerman was on board the flight, leading a trade delegation that was on its way to China.

He said that when the plane veered sharply, passengers started screaming and fearing for their lives.

The jet safely landed at the Porto Alegre airport in southern Brazil about 20 minutes after the incident with the man subdued in the rear of the plane, a police spokesman said.

The suspect was arrested by federal police and taken to a mental care facility.