LOS ANGELES — Just before dawn, 10 people met at a parking lot shared by a laundromat and coffee shop in South Central Los Angeles on what has become a daily mission: Look for immigration officers and warn people of their presence to try to prevent arrests. Bullhorns and sirens are ready for use.

“There’s raza that’s been detained,” Ron Gochez, founder of Union del Barrio’s Los Angeles chapter, said before they split up in five cars. “It seems like there’s more activity now. Let’s keep a close eye out.”

Working with other similar-size groups and using walkie-talkies, the Community Self-Defense Coalition, made up of more than 60 organizations, found nothing Thursday but appeared to have disrupted U.S. Immigration and Customs Enforcement operations earlier in the week. In Los Angeles and across the country, these tactics have been a thorn in ICE’s side as it tries to carry out President Donald Trump’s promise of mass deportations.

“Positive spotting right now of ICE agents at the Target in Alhambra,” Lupe Carrasco Cardona said in a live Instagram report Sunday from the Los Angeles suburb over a blaring siren to draw attention. She said at least six government vehicles were identified in the ICE operation.

Counter-ICE operations have had “a huge impact,” said John Fabbricatore, a former head of ICE’s enforcement and removal division in Denver.

“It’s dangerous for the officers because they are trying to get into a situation, maybe undercover, trying to make an arrest without alarming the neighborhood, and then these guys come out here with these bullhorns and they start yelling and screaming,” Fabbricatore said.

Advocates “go right to the edge” of a law against impeding federal law enforcement to avoid criminal prosecution, he said.

Advocates say they are exercising free speech and reminding people of their rights. ICE officers cannot forcibly enter a home without a judicial warrant, which they rarely have. Sophisticated “know-your- rights” campaigns urge people to stay inside and not open the door.

For years, including during Trump’s first administration, ICE has contended with advocates who rely on blast text messages, social media and bullhorns to spread the word.

Trump’s border czar Tom Homan was visibly angry after joining ICE officers who were met at apartments in the Denver area by activists who insulted them and used bullhorns to alert residents. He insisted word of the operation was leaked.

“The less people know about these operations, the better,” Homan said outside the White House after the Denver operation resulted in fewer arrests than expected.

ICE referred questions to the Homeland Security Department, which did not respond to questions about the advocates’ tactics and any activities in Los Angeles this week. The agency stopped releasing daily arrest figures, but Homan said last week that it made about 21,000 arrests. That’s an average of more than 600 a day, roughly double what President Joe Biden’s administration did in a 12-month period ending Sept. 30.

In the Los Angeles area, about 150 volunteers fanned out Sunday in response to rumors of ICE operations. Organizers said they spotted ICE in Alhambra and San Fernando, preventing arrests.

Volunteers meet before dawn, as many workers are heading to their jobs and when advocates believe ICE is most likely to move in. They zigzag through quiet residential streets and sleepy intersections, looking for double-parked vehicles, tinted windows and newer cars parked in red zones.

If they spot ICE officers, they hit record on their phones. They blare sirens. And through a megaphone, they announce that ICE is in the neighborhood. “They’re here.”

“We don’t use violence. We don’t break any laws. But we will do anything legally possible to defend our community,” Gochez said.

Back at the parking lot about two hours after Thursday’s mission started, the city was waking up. On the corner, a street vendor had set up her tamales stand.

“We just got a report right now that all of our patrols happening in San Diego, Escondido, California; Los Angeles, California; South Gate and Alhambra, everything is all clear right now,” said Gochez, before heading to his day job as a high school history teacher.