Mattis, who resigned in December 2018 over Trump's ordering of a full troop withdrawal from Syria, also voiced support for the demonstrators whose anti-racism rallies have roiled the country.

"Donald Trump is the first president in my lifetime who does not try to unite the American people -- does not even pretend to try," Mattis wrote in a blistering statement posted online by The Atlantic.

"Instead, he tries to divide us," added the retired Marine general, who had previously argued it would be inappropriate for him to criticize a sitting president.

"We are witnessing the consequences of three years without mature leadership," he stated.

Mattis described himself as "angry and appalled" after witnessing events of the last week, which saw Trump threaten a military crackdown on American citizens as nationwide protests turned violent in some cities.

The fury was ignited by the May 25 killing of George Floyd, a black man who suffocated beneath the knee of a white police officer, and whose agonizing death was filmed by bystanders.

The demonstrations have mostly been peaceful, but some have degenerated into violence and looting as night falls.

Mattis wrote that the protesters' call for equal justice was a "wholesome and unifying demand."

And he slammed the decision to use force to clear peaceful protesters from near the White House on Monday to allow Trump to pose for photographs at a nearby damaged church, calling it an "abuse of executive authority."

The photo op has become a lightning rod for criticism of Trump's handling of the crisis, with religious leaders, politicians, and onlookers around the country expressing outrage.

"When I joined the military, some 50 years ago, I swore an oath to support and defend the Constitution," Mattis stated.

"Never did I dream that troops taking that same oath would be ordered under any circumstance to violate the Constitutional rights of their fellow citizens -- much less to provide a bizarre photo op for the elected commander-in-chief, with military leadership standing alongside."

Trump dismissed Mattis with a tweet, rehashing his claim that he "essentially" fired his Pentagon chief.

"Probably the only thing Barack Obama and I have in common is that we both had the honor of firing Jim Mattis, the world's most overrated General," the president wrote.

Mattis was head of US Central Command when Obama fired him in 2013 over his hawkish views on Iran.

- 'We can unite without him' -

For months after Mattis resigned, he refused to criticize Trump publicly, insisting the military must remain apolitical.

Wednesday's statement appeared to signal that he no longer felt bound by that sentiment, as he called for solidarity -- with or without the president.

"We can unite without him, drawing on the strengths inherent in our civil society," Mattis wrote.

He also appeared to rebuke current Pentagon chief Mark Esper, without specifically naming him.

"We must reject any thinking of our cities as a 'battlespace' that our uniformed military is called upon to 'dominate,'" he said.

Esper faced criticism after telling US governors on Monday that they should "dominate the battlespace" to end the protests.

On Wednesday, the Pentagon chief backtracked, telling reporters: "In retrospect, I would use different wording."