DENVER — Immigrant communities across the U.S. braced Sunday for federal raids and detentions promised by President Donald Trump.

Trump confirmed the raids Friday after reports that thousands of immigrants would be arrested starting Sunday in major U.S. cities. Immigration reform advocates said the raids would tear apart families and sow further mistrust of the government. In preparation, advocates staffed hotlines, printed fliers with legal information and activated networks of volunteers to monitor and document Immigration and Customs Enforcement actions.

San Francisco Mayor London Breed said Friday that the city’s police would not cooperate with any ICE operations and that the city was gearing up to protect its immigrants. “If you want to come after them, you’re going to have to come through us,” she said.

In Denver and other cities, government human-service workers were on standby to find foster homes for any children left behind if their parents were detained and marked for deportation. In many cases, immigrants who lack legal permission to remain in the United States have minor children who are U.S. citizens. 

Immigration reform advocates expected that communities around Atlanta, Baltimore, Chicago, Denver, Houston, Los Angeles, Miami, New York and San Francisco would be targeted in the raids expected to last through at least Thursday. Trump said convicted criminals in the country illegally are being targeted first.

“It starts on Sunday and they’re going to take people out and they’re going to bring them back to their countries,” Trump told reporters at the White House on Friday. “We are focused on criminals as much as we can before we do anything else.”

The Trump administration argues the nation’s immigration laws have long been ignored, and that tougher enforcement is necessary because Democrats in Congress have failed to pass comprehensive immigration reform, while critics say the president’s hardline stance is aimed at bolstering his support among conservatives who make up his base. They called the raids heartless and unwarranted, citing the United States’ long history of welcoming refugees and immigrants.

The ACLU filed a lawsuit to stop the raids and subsequent deportations, arguing that many of the targeted people were unaware they were subject to what’s known as a “final order of removal” because federal officials did a poor job of proving accurate court dates and appointment updates.

“These refugees failed to appear because of massive bureaucratic errors and, in some cases, deliberate misdirection by immigration enforcement agencies,” the ACLU said in a lawsuit filed Thursday. “The agencies’ flagrant and widespread errors made it impossible for people to know when their hearings were being held.”