(CNN) US immigration authorities have detained some 680 undocumented immigrants in what a federal prosecutor described as a record-setting operation.

US Attorney for the Southern District of Mississippi Mike Hurst told reporters the arrests took place at seven sites in six different cities in Mississippi on Wednesday. The raids, he said, are "believed to be the largest single-state immigration enforcement operation in our nation's history."

Officials declined to provide details about what sites had been targeted, citing what they said was an ongoing criminal investigation. CNN affiliates reported the raids at food-processing plants throughout the state.

The arrests came as a result of administrative and criminal search warrants executed by special agents from Homeland Security Investigations, Hurst said.

"Today, through the hard work of these men and women, we are once again becoming a nation of laws," he said.

Immigration and Customs Enforcement has stepped up worksite enforcement since President Donald Trump took office, conducting a number of large-scale raids at food-processing plants and gardening centers in recent years.

"These are not new laws, nor is the enforcement of them new," acting ICE director Matt Albence said Wednesday.

"The arrests today were the result of a year-long criminal investigation. And the arrests and warrants that were executed today are just another step in that investigation."

Video shows girl begging to see her mom

A tense scene unfolded outside a plant in Morton, Mississippi, as friends and family members clamored for information and buses rumbled by to pick up large groups of detained immigrants and take them away.

A Facebook video broadcast live at the scene showed an 11-year-old girl sobbing and begging an officer for a chance to see her mother as bystanders tried to comfort her.

Elizabeth Iraheta told CNN she shared the video so people would see what was happening at the chicken plant where she said she's worked legally for 19 years. It was devastating, Iraheta said, to see family and friends suffering "just for coming to work hard in this country, and to see so many families separated."

Iraheta's video shows an officer noting that the mother is being processed "because she doesn't have papers to be here legally," then later telling the crying girl that her mother would be released and wouldn't be deported. Eventually, Iraheta said, the girl's mother was able to see her before she was taken away.

But Iraheta said she isn't sure what will happen to the girl's mother, how many people were detained at the plant where she works or what will happen to them now.

"Today was the first day of school. All the parents went to take their kids to school," Iraheta said. "Now their kids maybe are alone. "

Albence, the acting ICE director, told reporters Wednesday that in the past the agency has worked with school liaisons to help "find placement" for children when their parents are detained.

"Most of the time, they're placed with another family member," he said, adding that each case would be evaluated and handled individually.

"Some of the parents that were arrested will be released and placed on an ankle monitor throughout (their immigration) proceedings," he said.

Mayor: 'What happens to the children?'

Several local officials criticized the operation.

Jackson Mayor Chokwe Antar Lumumba called the raids "dehumanizing and ineffective," issuing a statement calling for local houses of worship "to become sanctuaries for our immigrant neighbors and protect them from potential harm."

William Truly Jr., the mayor of Canton, Mississippi, told reporters outside a processing plant owned by Peco Foods Inc. that he was concerned about the impact the arrests would have on the local economy.

"I recognize that ICE comes under the Department of Homeland Security, and this is an order of the United States. There's nothing I don't think anybody can do about it," he said. "But my main concern is now, what happens to the children?"

Fears are running high in immigrant communities

Wednesday's arrests come as anxiety is running high in many immigrant communities days after a gunman who apparently espoused anti-immigrant views killed 22 people at a Walmart in El Paso, Texas.

Hurst acknowledged the recent tragedy in response to a reporter's question Wednesday, but said the operation's timing came as a result of careful planning in a lengthy investigation.

"This operation began over a year ago. You don't bring over 650 special agents from across the country into the Southern District of Mississippi in a matter of three days without preparation for months and months and months," he said. "So while the tragedies this weekend around the country are horrific, this operation had been planned way before that, and we intended to carry it out."

In a statement provided to CNN affiliate WAPT , a company that owns several facilities where arrests occurred Wednesday said it is cooperating with investigators.

"We are fully cooperating with the authorities in their investigation and are navigating a potential disruption of operations," Peco Foods Inc. said in a statement. "We adhere strongly to all local, state and federal laws including utilizing the government-based E-Verify program which screens new hires through the Social Security Administration as well as the Department of Homeland Security for compliance."