(Updates with quotes)

GRENOBLE, France (AFP)--French workers Tuesday detained four managers from U.S. firm Caterpillar Inc. (CAT) at a plant in the southern city of Grenoble in protest at plans to slash hundreds of jobs, a union official said.

The employees barricaded their bosses in a management office at around 10:30 am (0830 GMT) to demand fresh negotiations on laying off 733 workers at the factory, said union official Benoit Nicolas.

"We are holding them in the director's office," said Nicolas. "They are a little stunned."

Factory director Nicolas Polutnick was being held along with the human resources director, the head of personnel and a manager but Nicolas said they would not be held for long.

"We do plan to let them go," he said, adding that a deal was within reach.

It was the third time in March that executives had been held by French workers outraged at learning about job losses.