The show of force comes as huge rallies took the streets of America from coast to coast to protest against President Donald Trump.

A 17-mile (27-kilometer) coastal stretch of the I-5 interstate between Los Angeles and San Diego will be affected while the Marine Corps fires live rounds over the route to mark its 250th anniversary in an event being attended by Vice President JD Vance.

"The President is putting his ego over responsibility with this disregard for public safety," California Governor Gavin Newsom said.

"Firing live rounds over a busy highway isn't just wrong - it's dangerous."

On Saturday, AFP journalists travelling towards the interstate, which runs from the Canadian border to the Mexican frontier, saw signs that blared "Live weapons over freeway."

In a statement, the Marine Corps insisted there would be no risk to the public from the ordnance, which it said would showcase "the strength and unity of the Navy-Marine Corps team... ensuring we remain ready to defend the Homeland and our Nation's interests abroad."

"Artillery pieces have historically been fired during routine training from land-based artillery firing points west of the I-5 into impact areas east of the interstate within existing safety protocols and without the need to close the route," the statement said.

"This is an established and safe practice."

Saturday's "No Kings" rallies are being held to protest what participants see as the overreach of Trump's government.

"Using our military to intimidate people you disagree with isn't strength - it's reckless, it's disrespectful, and it's beneath the office he holds," Newsom said.

The dispute is the latest involving the military between liberal California and the White House.

In June, Los Angeles became the first of several cities to which Trump has sent National Guard troops after unruly protests over immigration raids.

The deployment, which also included hundreds of Marines, was criticized as heavy-handed and unnecessary by local officials, including Newsom. They insisted the relatively small demonstrations could easily be handled by city and state law enforcement.