Retired Marine Gen. Jim Mattis, who resigned as President Trump's defense secretary nearly a year and a half ago over policy differences, has issued an extraordinary critique of the White House's handling of nationwide unrest, saying Trump has sought to divide Americans and warning against "militarizing our response" to the protests.

"When I joined the military, some 50 years ago, I swore an oath to support and defend the Constitution," Mattis said in a statement published in The Atlantic.

"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," Mattis wrote, referring to a series of events on Monday in which peaceful protesters were physically cleared from around the White House to make way for an appearance by the president, Bible in hand, outside a historic church.

Mattis called Monday's actions "an abuse of executive authority."

The broadside came on the same day that current Defense Secretary Mark Esper said at a Pentagon briefing that he opposed invoking the Insurrection Act — which allows the president to deploy the military within the United States — something Trump had threatened to do in a televised speech on Monday.

Earlier this week, Esper had referred to unrest in the streets as a "battlespace," and Trump urged governors to use law enforcement and the National Guard to "dominate" the protesters.

"We must reject any thinking of our cities as a 'battlespace' that our uniformed military is called upon to 'dominate,' " Mattis wrote. "At home, we should use our military only when requested to do so, on very rare occasions, by state governors."