The workers from seven agricultural processing plants were arrested on Wednesday for allegedly not having proper documentation to be in the US.
At a press conference, officials from ICE's Homeland Security Investigations and the Attorney's Office for the Southern District of Mississippi said that of the 680 people arrested in raids, 300 people, including pregnant women and juveniles, were released on Thursday on "humanitarian grounds", the US media reported.
Those released on their own recognizance were served with notices and at some point will have to appear before immigration judges. Others were transported to detention facilities in Louisiana and Mississippi, said the officials.
Pictures emerged of children crying after being separated from their parents. Democrats and rights groups have condemned the arrests as "cruel".
But officials said those detained in the operation were asked if they had children at school or at child care who needed to be picked up. Detainees were offered cellphones so they could make the necessary arrangements for their children, reports say.
President Donald Trump had announced an immigration crackdown in June, saying "millions of illegal aliens who (had) found their way into the US" would be removed.
ICE spokesman Bryan Cox told the BBC that those who were not released will be moved to the agency's detention facility and held there.
The ICE did not share details about the nationalities of those detained, but the Mexican government reportedly sent consular staff to the area to help any of their nationals who may be involved.
Nora Preciado, a supervising attorney at the National Immigration Law Centre, told the BBC that, in many workplace raids, "the ICE often singles out people in a discriminatory fashion by focusing only on the Latino workers, and there are many incidents of excessive force during the detention and arrests".
She said that research shows raids like this have a "harmful impact on safety, educational success, social and behavioural well-being and overall health of children in immigrant families".
( With inputs from IANS )