Gov. Gavin Newsom of California sharply criticized Defense Secretary Pete Hegseth, who said that active duty Marines could be mobilized as part of the federal government’s response to protests against immigration raids in the Los Angeles area.

Mr. Hegseth’s suggestion came on Saturday after President Trump ordered at least 2,000 National Guard members to assist immigration agents following two days of clashes with demonstrators. Some of the demonstrations have been unruly, but local officials had not asked for federal assistance and Mr. Trump issued the order under a rarely used law to bypass Mr. Newsom’s authority.

Mr. Hegseth welcomed the president’s decision as “common sense” and said that Marines at Camp Pendleton, about 100 miles south of Los Angeles, were on high alert. They could be deployed to deal with any violence, he said in a post on X, formerly Twitter.

Raising and lowering the alert status for active duty troops is within the purview of the defense secretary, but actually deploying those troops can be done only by the president. To do so, Mr. Trump would need to invoke the Insurrection Act because deploying active duty troops on American streets is a violation of the Posse Comitatus Act, which prohibits direct involvement of federal troops in law enforcement.

That did not stop Mr. Hegseth from threatening to deploy Marines, though he did so from his personal social media account and not his official secretary of defense account.

Mr. Newsom said in a post on social media overnight that Mr. Hegseth was “threatening to deploy active-duty Marines on American soil against its own citizens.” He added, “This is deranged behavior.”