The added police presence, in addition to Chrysler security, comes as the plant prepares to eliminate its third shift Friday.
"We're aware of rumors of potentially threatening or disruptive behavior," said Chrysler spokeswoman Michele Tinson. "We do have a police presence there in order to offset any negative behavior in relation to the elimination of the third shift."
As part of cuts announced in November, 1,069 workers will be laid off this week.
Local media reports say the threats reference a shooting that killed six last month at nearby Northern Illinois University.
Police Thursday were patrolling the plant and its parking lots, checking workers' bags as they enter the factory, said Andy DeRose, a Belvidere worker.
Tensions at the plant are high and there's talk of workers vandalizing cars in production, he said.
DeRose said he is fearful for his safety.
"You don't know what's going to happen," he said. "If someone wanted to bring in a gun, they could have done it weeks ago." Detroit News