The mood in Los Angeles was tense this afternoon, as the city braced for a fourth day of confrontations between law enforcement and demonstrators protesting the Trump administration’s immigration crackdown. Over the weekend, the authorities responded to pockets of violence with crowd-control munitions, tear gas and flash-bang grenades.

President Trump responded by sidestepping the state’s Democratic governor and deploying 2,000 National Guard members to the city. He portrayed the demonstrations as an existential threat to the country, and called the protesters “insurrectionists,” in what may become a rationale for him to invoke the Insurrection Act.

Defense officials also said that a battalion of 500 Marines was “being mobilized” to help quell protests in Los Angeles.

California officials argued that Trump’s decision to deploy troops had violated the Constitution and inflamed tensions. The state’s governor, Gavin Newsom, said that the state would sue the president.

The Times spoke with protesters, some of whom said they were first- or second-generation immigrants showing support for neighbors or family members. Some flew the Mexican flag to signify pride in their roots and solidarity with people being targeted for deportation. See photos of the protests.

For more on the protests:

• There were signs that the tension might spread to other cities. In San Francisco, the police said that more than 150 arrests had been made at solidarity protests late last night.

• Several journalists, including one of my colleagues, were injured while covering the protests.

• Waymo, the robot taxi company, halted service after its vehicles were set on fire.

• Wonder how the National Guard works? Here’s a short explanation, as well as a look at the last time a president bypassed a governor to deploy them.