About 12,700 workers from one factory each of the three biggest automakers in the United States walked off their job Friday in a historic strike.

The strikers marked the walkout outside a Detroit-area Ford plant with rowdy honking and cheers at the arrival of the United Auto Workers' union leader Shawn Fain

"Tonight, for the first time in our history, we will strike all three of the Big Three at once," Fain said in a webcast shortly before the 14 September deadline to sign a new contract with General Motors, Ford and Stellantis.

Fain said he had hoped to avoid a strike, but blamed the companies for waiting too long to begin serious negotiations.

"We're going to be out here until we get our share of economic justice," the union president said. "And it doesn't matter how long it takes."

Fain said the union would strike at a GM factory in Wentzville, Missouri; a Stellantis facility in Toledo, Ohio; and a Ford plant in Wayne, Michigan, but only the final assembly and paint operations.

The 12,700 who walked out represent only a fraction of the 150,000 auto workers represented by UAW.

The union demands include a 40-percent hike in wages, which Fain has said is needed to match rises in CEO pay.

Other sticking points include raising pay and benefits for junior employees to match the level of more seasoned workers, who currently make a top rate of around $32 an hour.

Many hourly workers say the auto giants must produce significantly better packages to make up for meager wages and benefit cuts after the 2008 financial crisis, when both GM and Chrysler, now part of Stellantis, underwent bankruptcy reorganizations.

All three companies have been highly profitable in recent years.

On Thursday, GM upped its offer, lifting a proposed wage increase to 20 percent. The company had previously proposed an 18 percent rise, according to the UAW.

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."