Workers said they were creating a blockade, stopping trucks carrying supplies from entering the plant.
Canadian Prime Minister Justin Trudeau on Monday expressed his "deep disappointment" in General Motors' decision to close its Oshawa plant, a move Canadian officials only learned about on Sunday and which led workers to walk off the job on Monday.
"We believe in an all-electric future", Ms. Barra said.
Chief Executive Officer Mary Barra has said the move will free up as much as US$6 billion annually to spend on electric vehicle and autonomous driving technology.
Two transmission plants, in Warren, Mich., and in Baltimore, also will close. The Oshawa Assembly Plant opened in 1953.
Production began at the Oshawa plant on November 7, 1953, and in the 1980s the plant employed roughly 23,000 people.
In addition, the assembly plant in Gunsan, Korea, General Motors will cease the operations of two additional plants outside North America by the end of 2019.
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"I've never seen anything like it".
The Oshawa Assembly Plant employs 2,522 workers with Unifor Local 222, according to GM's website.
Unifor, the union representing more than 2,500 workers at the plant, said it has been informed that there is no product allocated to the Oshawa plant past December 2019.
"Are we disappointed? Yeah, we're disappointed in GM", Ontario Premier Doug Ford told reporters in Toronto, pointing out that the company benefited from billions of dollars in life-support from the government during the economic downturn of a decade ago.
Former Canadian Auto Workers head Buzz Hargrove told the Sun news of the closure left him reeling.
The salaried work force restructuring includes cutting 15 percent of its 54,000 salaried employees in North America - more than 8,000 including cutting global executives by 25 percent. "We recognize the need to stay in front of changing market conditions and customer preferences to position our company for long-term success".
Trump criticized the closures while speaking to reporters Monday afternoon and said he spoke to Barra.
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GM was swept up in the financial crisis of 2008-09, and received bailout loans worth US$60-billion from the governments of Canada, Ontario and the United States.
The US$6 billion in savings includes cost reductions of US$4.5 billion and lower capital expenditure annually of nearly US$1.5 billion.
GM's actions are meant to increase the automaker's profits and strengthen its core business, while it doubles investment in autonomous and battery electric vehicles by 2020 - a year before the automaker is expected to launch an all-new profitable electric vehicle platform.
In an emailed statement to CTV Toronto, GM spokesperson David Paterson said the company "won't be commenting. on speculation".
"Based on commitments made during 2016 contract negotiations, Unifor does not accept this announcement and is immediately calling on GM to live up to the spirit of that agreement". Last month, GM rival Ford Motor.
The company says 5,901 hourly employees and 804 salaried employees work at these plants.
He adds that about half of the affected employees in Oshawa are eligible for an early retirement and says the facility near Highway 401 will remain as the company's Canadian headquarters.
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The loan also appears to be attributed to a bankrupt version of the firm that was split off from a viable version of GM that year.