More than 10,000 staff at three of biggest US carmakers have started strike action, their union says.
Work has been stopped at three plants owned by General Motors (GM), Ford and Stellantis.
It came after labour contracts expired on Thursday night. The United Autoworkers Union (UAW) said the firms had not put forward acceptable offers.
The fight threatens to trigger higher prices for buyers and major disruption for the motor industry giants.
The UAW’s president Shawn Fain told the BBC it was now up to the companies to resolve the dispute.
“When they start taking care of their workers it will end,” he said.
The strike started at midnight eastern time (04:00 GMT) at GM’s Wentzville, Missouri mid-size truck plant, Ford’s Bronco plant in Michigan and the Jeep plant in Toledo, Ohio owned by Stellantis.
The plants are critical to the production of some of the “Detroit Three’s” most profitable vehicles.
Other facilities will continue to operate, the UAW said but it did not rule out broadening the strikes beyond the initial three targets.
With the deadline looming on Thursday, the White House said that President Joe Biden had spoken on the phone with Mr Fain about the negotiations but provided no further details.
The union had sought a 40% pay increase for its roughly 140,000 members over four years, noting a comparable rise in pay for company leaders.
Other demands included:
a four-day working week
the return of automatic pay increases tied to inflation
stricter limits on how long workers can be considered “temporary” staff who do not receive union benefits
The UAW’s proposals would more than double its US labour costs, Ford said in a statement.
Last month, 97% of the union’s members voted to authorise a strike.
Workers said the companies could afford to be more generous after years of record profits.