Chanting protesters briefly blocked a bus leaving a migrant detention center in McAllen, Texas, on Saturday afternoon.
Protesters stood in front of the bus and yelled, "Set the children free" and "Shame on you" at Border Patrol officers. The Border Patrol ended up surrounding the bus so it could back up and go out the other end of the street.
CNN reporters could see children through the darkened windows, and a protester told CNN she also saw children through the windows. She said some of the children waved at her and she told them, "You are not alone" in Spanish.
"It was very difficult to see," said Denise Benavides of Dallas, who said she's a member of the League of United Latin American Citizens.
Benavides said she didn't know where the bus was going but "that's something we'll look into -- what's going on and where are they taking these children."
Uniformed officers arrived to calm the situation. McAllen police told CNN nobody was arrested.
The vehicle's destination was unclear.
A US Customs and Border Patrol spokesperson said the bus carried "family groups" being transferred into the custody of Immigration and Customs Enforcement. When asked about the bus's destination, an ICE spokesman recommended contacting Health & Human Services or CBP. HHS said to ask ICE.
Many of the at least 2,300 children separated from their undocumented parents since May are in far-flung shelters and foster homes nationwide -- hundreds of miles away from the southern border.
In many cases, the parents don't know where their children are being held, the parents' lawyers say.
On Wednesday, President Trump signed an executive order reversing his policy and allowing parents and their children to stay together.
US Rep. Jackie Speier, a Democrat from California, and other members of Congress toured the McAllen Customs Border Protection Facility on Saturday. She told CNN she's seen no evidence of a method to reunify parents and children.
"The staff at all the facilities are really operating with no policies," Speier said. "The President, you know, signs an executive order, and then washes his hands of it. That is unacceptable.
"This is on his watch. This is his process. This is his policy he put in place. If you're going to undo it, then you truly have to undo it by making sure that you match every child with every parent."
More than 1,100 immigrants -- including children -- were being held at McAllen when reporters were allowed inside on June 17.