WSJ's Katherine Sayre reports from downtown Los Angeles where thousands are taking part in anti-Trump demonstrations outside City Hall.

To some Angelenos, the notion that their city was being invaded by Mexico might sound like a fantastical twist on the history of U.S.-Mexico military relations.

"I urge the authorities to respect the right to peaceful assembly and to uphold human rights in law enforcement, including by refraining from any resort to military force when civilian authorities ar...