The move is criticized by Democrats as some mayors say they will not cooperate

Fear gripped migrant communities in the US and advocate groups were on heightened alert as federal agents prepared to carry out raids in major cities on Sunday to deport thousands of undocumented people.

US President Donald Trump insisted that the raids will focus primarily on deporting criminals, though he acknowledged that the agents will be targeting anyone who crossed the southern border illegally.

"It starts on Sunday, and they're going to take people out and they're going to bring them back to their countries," the president said. "Or they're going to take criminals out, put them in prison, or put them in prison in the countries they came from."

The raids by US Immigration and Customs Enforcement, or ICE, are expected to take place in Atlanta, Baltimore, Chicago, Denver, Houston, Los Angeles, Miami, New York and San Francisco. New Orleans city officials said that they had confirmed with ICE immigration enforcement actions would be temporarily suspended through the weekend in areas of Louisiana affected by Tropical Storm Barry.

Most of the United States' 10.7 million unauthorized immigrants live in just 20 major metropolitan areas, with the largest populations in New York, Los Angeles, Houston and Dallas-Fort Worth, according to Pew Research Center.

Local ICE branches could start the raids sooner or later than Sunday, officials said, and last at least through Thursday.

Amy Lin, immigrant rights program manager at Chinese for Affirmative Action, a San Francisco-based civil rights organization, told China Daily on Friday that "we have seen ICE operations in the Bay Area since last week".

"Though the undocumented immigrants will be directly impacted, all people of color are targeted," she said. "The raids are politically motivated and meant to terrorize the immigrants and try to deter those migrants from coming into the US."

Hundreds of thousands of migrants have crossed the southern border this year, many fleeing violence and poverty to seek asylum in the US.

Arrests at the border dropped by 28 percent in June, according to the Department of Homeland Security, the first time this year that the number of border crossings declined.

The department said earlier this month that 104,344 arrests occurred in June, down from 144,278 in May－the highest monthly total in 13 years.

The nationwide immigration enforcement operation had been announced and subsequently postponed by Trump late last month over protests from Democratic lawmakers and others.

Then ICE agents were targeting at least 2,000 immigrants. Federal immigration authorities said they hoped the new effort would sweep up at least 2,000 undocumented migrants across the country.

The raids have been heavily criticized by Democratic lawmakers and some mayors who say they will not cooperate with the ICE.

The looming raids could have long-lasting impacts on immigrant communities, said UC Davis School of Law dean Kevin Johnson:

"There's real damage to communities. I'm not saying everybody's staying home, but people are fearful of leaving their homes, fearful of going to work, fearful of going to school, fearful of going to the doctor, fearful of going to the DMV."

Protest vigils against the raids have started last week in many cities and are being held this weekend.

In Los Angeles, Cristel Martinez spoke at a vigil organized by advocates in front of Metropolitan Detention Center on Friday night. She said she was born in Honduras, moved to the US when she was nine and now is an immigration attorney.

"People are terrified knowing that they are going to have raids this weekend," she told China Daily. "I am just reminding them that they have rights, that they don't have to open the doors, and they have the right to not say anything, and they have the right to an attorney as well."

That is the same message being spread by advocacy groups in fliers and on social media across the country to undocumented families: Don't open your door if ICE agents show up. They are not legally allowed to forcibly enter a house or business without a warrant signed by a judge.

In San Francisco, activists have set up a network of hotlines to confirm reports about raids and readied volunteer lawyers to assist detainees in court.

"It is unconscionable that the federal administration is continuing to target innocent immigrant families with raids that are designed to inflict as much fear and pain as possible," said Mayor London Breed in a statement on Friday.

Lia Zhu in San Francisco and Yinmeng Liu in Los Angeles contributed to this story.