A mass rally was scheduled for Friday afternoon in downtown Detroit.
"We will show our strength and unity on the first day of this historic action," Fain said. "All options remain on the table."
The work stoppage marks the first strike at the Detroit automakers since workers walked out on GM in 2019.
The UAW's demands include a 36% pay increase across a four-year contract; pension benefits for all employees; limited use of temporary workers; more paid time off, including a four-day workweek; and more job protections, including the right to strike over plant closings.
With talks at an impasse on Thursday, leaders at Ford, General Motors and Stellantis (formerly Fiat Chrysler) said they had made multiple offers to the UAW in recent weeks in hopes of inking a new deal for the union's 145,000 workers.
"I think they're preparing for a historic strike with all three companies," Ford CEO Jim Farley told CBS News earlier Thursday.
Ford said in a later statement that, "At 8 p.m. this evening at Solidarity House in Detroit, the United Auto Workers presented its first substantive counterproposal to Ford a few hours from the expiration of the current four-year collective bargain agreement."
After the strike was underway, Stellantis said it was "extremely disappointed by the UAW leadership's refusal to engage in a responsible manner to reach a fair agreement in the best interest of our employees, their families and our customers. We immediately put the Company in contingency mode and will take all the appropriate structural decisions to protect our North American operations and the Company."
And GM said, "We are disappointed by the UAW leadership's actions, despite the unprecedented economic package GM put on the table, including historic wage increases and manufacturing commitments. We will continue to bargain in good faith with the union to reach an agreement as quickly as possible for the benefit of our team members, customers, suppliers and communities across the U.S. In the meantime, our priority is the safety of our workforce."
Although the Big Three have been unwilling to fulfill all of the UAW's demands, they contend they've made reasonable counteroffers and are willing to negotiate further. In outlining their position, automaker officials say that they're under enormous pressure to keep costs and car prices low in order to compete with Tesla and foreign car makers, especially as the companies compete for a stake in the growing electric vehicle market.
"What their initial offer was, is to pay our hourly workers about $300,000 each, and to work four days," Farley said of UAW Thursday. "That would basically put our company out of business."
Although Fain acknowledged that the automakers had upped their wage offers, the proposals remain inadequate, he said. Ford has offered 20% over 4.5 years, while GM and Stellantis offered 18% and 17.5% over four years, respectively.
The strike could cause a surge in car prices, result in $5.6 billion in economic losses for the automakers, according to one forecast and reduce the nation's GDP by as much as 0.3%, according to Oxford Economics.