Updated at 5:20 p.m. with details from the protests in McAllen

McALLEN — Calls to end the Trump administration's "zero-tolerance" crackdown on migrant families escalated Saturday when protesters from Dallas and other cities briefly blocked an immigration bus, while other civil rights leaders launched a nationwide fast.

A rally at a U.S. Customs and Border Protection station took a swift turn when hundreds of protesters saw a bus arriving with child migrants — and they blocked its passage.

"Set them free!" the demonstrators chanted, as police arrived to disperse the crowd.

The Rev. Peter Johnson, a longtime civil rights activist in Dallas, led a group in prayer, asking for strength "to help America find America."

Reverend Peter Johnson broke in tears after a bus full of children left the @ICEgov detention center in #McAllen “If they don’t let us in is because something really awful is going on in there” pic.twitter.com/X8Qs4dSLQz — Jenny Manrique (@JennyManriqueC) June 23, 2018

Johnson, who served on the staff of Martin Luther King Jr., got choked up after seeing the young passengers peering out the tinted glass.

"We saw the eyes of these young children through the windows ," he said.

Some protesters turned their anger on the border agents and shouted, “Shame on you!"

This bus carried immigrant children housed at the McAllen, Texas center. It was obstructed by our protests as we demanded immediate and safe reunification of children with their families. #freethechildren pic.twitter.com/nqprxjhQuX — Leo Lopez III M.D. (@LeoLopez3MD) June 23, 2018

"It was heartbreaking to see the kids," said Esseiny Alanis, a Dallas teacher at the rally. Alanis said the children looked frightened.

Dallas-area imam Omar Suleiman joined in the McAllen protest, saying those responsible for the crackdown should reconsider their actions.

"Anyone who participated at any capacity in this needs to look at the mirror and ask themselves what they are doing," Suleiman said. "I get they are not the ones making the policies, but this is inhumane."

Today in McAllen, TX: A caged police bus transporting children seeking amnesty. In this pic Sh.@omarsuleiman504 places a palm on a child's hand and prays.



Look closely to see the tiny hands of the children pressed against the tinted windows! #freethechildren #FamilySeparation pic.twitter.com/Hpv2mb5CIQ — Mohamed Soltan | محمد سلطان (@soltanlife) June 23, 2018

Saturday evening, federal immigration officials had little comment on the protest at the McAllen Border Patrol station.

But Customs and Border Protection spokeswoman Yolanda Choates said they were transferring "family groups" from the central processing center in McAllen to the custody of Immigration and Customs Enforcement, a separate agency within the Department of Homeland Security.

The delegation from Dallas came in two buses that left early Saturday morning. They asked permission to visit shelters for young children but were denied on Friday, said Domingo García, president of a local chapter of the League of United Latin American Citizens, or LULAC.

"We passed all the background checks," García said. "We were aware we can't film or record anything, but at the end the names approved were denied" by the Office of Refugee Resettlement, a Health and Human Services Department agency that provides housing for children.

@SisterNormaCCRG leads the prayer to kickoff the 24-hour, 24-day fast for immigrant families. They will be carrying little replicas of the medal that #CesarChavez used during his fast in the 60’s when supporting farmer workers rights. pic.twitter.com/6lR1TGKClS — Jenny Manrique (@JennyManriqueC) June 23, 2018

At Archer Park near downtown McAllen, civil rights leaders Dolores Huerta and Kerry Kennedy called for a national fast on behalf of migrant families caught in the White House crackdown.

Dozens of people joined the "Break Bread, Not Families" rally wearing red T-shirts and holding signs with messages that included "Mother and children do not belong in detention," "Stop deportations" and "No hate! No fear!"

Kennedy, the daughter of Robert F. Kennedy, took aim at the new policies that level criminal misdemeanor charges against immigrants accused of illegally entering the U.S. The law has been on the books for decades, but immigrants are far more likely to face deportation hearings for a civil offense in the nation's immigration courts.

People brought out large signs in support of the Break Bread Not Families rally at Archer Park in McAllen, Texas, Saturday, June 23, 2018. The rally was hosted by LUPE (La Union Del Pueblo Entero) to protest the treatment of families crossing into the United States. (Tom Fox/The Dallas Morning News) (Tom Fox / Staff Photographer)

Kennedy said the migrants are fleeing violence in El Salvador, Honduras and Guatemala and should be considered lawful asylum-seekers. She called the leveling of misdemeanor charges "manufactured criminality."

Organizers said they wanted people to commit to fasting for 24 hours and then get a commitment from others to do the same over a 24-day period. Each day is meant to symbolize 100 migrant children among the almost 2,400 who federal officials say have been separated from their parents.

A large black-and-white framed photo of RFK and labor leader Cesar Chavez was displayed at the event. Many chanted, "Si, se puede," or "Yes, it can be done," the mantra of the farm worker movement that was founded by Chavez, Huerta and others.

Huerta told the crowd the fast would spread in popularity and help shine a light on the immigration crisis.

"This fast is going to make the difference," she said. "I am sure it is going to jump borders."

@KerryKennedyRFK @DoloresHuerta lead the 24-hour, 24 day fast and prayer chain for the 2,400 children separated from their parents. Organizations like #LUPE #TexasCivilRightsProject are supporting their efforts in the Valley pic.twitter.com/bRgQHgV06P — Jenny Manrique (@JennyManriqueC) June 23, 2018

Huerta said she would start her fasting on Sunday when she takes the zero-tolerance protest to Tornillo, outside El Paso.

"Cesar Chavez did [fasts] alongside Robert F. Kennedy to protest workers' rights," she said. "Now we need to tell this administration that the indefinite detention of families is not the solution to the problems in Central America."

1 / 12Dallas area religious leaders hold a prayer circle before departing the US Border Patrol Processing Center where they protested in McAllen, Texas, Saturday, June 23, 2018. The group was there to protest the border policies for immigrants crossing into the United States from Mexico. (Tom Fox / Staff Photographer) 2 / 12The Rev. Peter Johnson grasps his bible and he an other Dallas religious leaders pray outside the US Border Patrol Processing Center in McAllen, Texas, Saturday, June 23, 2018. The group was there to protest the border policies for immigrants crossing into the United States from Mexico.(Tom Fox / Staff Photographer) 3 / 12Dallas lawyer Domingo Garcia (left) leads a group of protesters outside the US Border Patrol Processing Center in McAllen, Texas, Saturday, June 23, 2018. The group was there to protest the border policies for immigrants crossing into the United States from Mexico.(Tom Fox / Staff Photographer) 4 / 12Nahela Morales of Richardson, Texas is comforted by her son Andrew, 13, as they and other Dallas area folks protested outside the US Border Patrol Processing Center in McAllen, Texas, Saturday, June 23, 2018. Nahela, a single mother, was emotional upset by the treatment of children that were being separated from their families.(Tom Fox / Staff Photographer) 5 / 12McAllen police officers were called to the US Border Patrol Processing Center in McAllen, Texas after protesters attempted to stop a bus from leaving the facility, Saturday, June 23, 2018. (Tom Fox / Staff Photographer) 6 / 12Mexican-American labor leader and civil rights activist Dolores Huerta (center) joined human right activist Kerry Kennedy (right, daughter of Robert F. Kennedy) in speeches during a Break Bread Not Families rally at Archer Park in McAllen, Texas, Saturday, June 23, 2018. Joining them was Efren Olivares (left), a Texas Ccivil rights attorney. Kennedy was there to start a 24-day chain fast on behalf of the 2,400 children separated at the Mexican border. The rally was hosted by LUP (La Union Del Pueblo Entero) to protest the treatment of families crossing into the United States. (Tom Fox / Staff Photographer) 7 / 12Longtime activist Dolores Huerta (right) leads people assembled for the Break Bread Not Families rally in a chant at Archer Park in McAllen, Texas, Saturday, June 23, 2018. The rally was hosted by LUPE (La Union Del Pueblo Entero) to protest the treatment of families crossing into the United States.(Tom Fox / Staff Photographer) 8 / 12A child is reflected racing past a black and white photo of Robert F. Kennedy and civil rights activist Cesar Chavez (right) during a Break Bread Not Families rally at Archer Park in McAllen, Texas, Saturday, June 23, 2018. Robert's daughter and activist Kerry Kennedy spoke at the rally hosted by LUPE (La Union Del Pueblo Entero). (Tom Fox / Staff Photographer) 9 / 12Sister Norma Pimentel leads a group, mostly from LUPE (La Union Del Pueblo Entero), in prayer as they close out the Break Bread Not Families rally at Archer Park in McAllen, Texas, Saturday, June 23, 2018. Human rights activist Kerry Kennedy kicked off a 24-day chain hunger fast in support of the 2,400 children separated from their families . (Tom Fox / Staff Photographer) 10 / 12People representing LUPE (La Union Del Pueblo Entero) wave United Farm Workers flags during a Break Bread Not Families rally at Archer Park in McAllen, Texas, Saturday, June 23, 2018. Human rights activist Kerry Kennedy kicked off a 24-day chain hunger fast in support of the 2,400 children separated from their families .(Tom Fox / Staff Photographer) 11 / 12People assembled in the searing heat to hear Break Bread Not Families speakers, including Kerry Kennedy and Dolores Huerta, at Archer Park in McAllen, Texas, Saturday, June 23, 2018. The rally was hosted by LUPE (La Union Del Pueblo Entero) to protest the treatment of families crossing into the United States.(Tom Fox / Staff Photographer) 12 / 12Imam Omar Suleiman of Irving, Texas leads a group of Dallas area protestors in prayer outside the US Border Patrol Processing Center in McAllen, Texas, Saturday, June 23, 2018. (Tom Fox / Staff Photographer)

Tania Chavez, an unauthorized immigrant who lives in the area and came to the U.S. as a 14-year-old, said she would fast for the migrant children.

"These have been really difficult days," Chavez said. "Every time they tell us we cannot find our children, it breaks our hearts."

Chavez works with the nonprofit immigrant organization La Union del Pueblo Entero, or LUPE. LUPE was a sponsor of the Saturday event.

Fast organizers asked that any money saved during a daylong fast can be sent as a donation to LUPE at https://lupenet.org, to the RFK Human Rights Foundation at www.rfkhumanrights.org or to the Texas Civil Rights Project, https://texascivilrightsproject.org.

The Texas Civil Rights Project is working closely with immigrant families who have relatives detained in the crackdown.

The White House announced plans in April to prosecute all immigrants caught along the southwest border with illegally entering the country. Parents were jailed and their children were taken to government-contracted shelters.

Now, after an executive order by President Donald Trump, the administration says it will continue with prosecutions and seek to detain families together during their immigration proceedings.

Confusion has ensued, with parents left searching for their children.

Immigration officials have said they could seek up to 15,000 beds in family detention facilities, and the Pentagon is drawing up plans to house as many as 20,000 unaccompanied immigrant children on military bases.

Those proposals have also sparked an outcry from women's and children's advocates who say children don't belong in jail.

In McAllen, @KerryKennedyRFK calls for end to “manufactured criminality” of migrants now caught in the #zerotolerance crackdown. Immigrants often face only civil proceedings. #familyseparation pic.twitter.com/ZcUSR6CmKf — Dianne Solis (@disolis) June 23, 2018

Dr. Amy Cohen, a child psychiatrist from the Los Angeles area, joined the fast, too. She's volunteering temporarily at the Humanitarian Respite Center run by Catholic Charities of the Rio Grande Valley.

She said the children she has seen have been traumatized by seeing their parents apprehended by border agents and kept behind metal fencing that she said looks like cages.

"Those kids are really suffering," Cohen said.

Boys, especially, are having problems processing what has happened to their parents, Cohen said. Some children are so distressed they vomit. Others are obsessed by the possibility they will lose a parent.

Leroy Peña was among the delegation who came to #McAllen from Dallas to protest the detention of childrens. “These are our children too, we can’t let @ICEgov to act with impunity”. The protesters run into a bus full of children and formed a human chain. pic.twitter.com/m1IJ4CnR03 — Jenny Manrique (@JennyManriqueC) June 23, 2018

As the vigil and rally ended in Archer Park, children began writing letters of support to the migrant children on red and purple paper. Roxanne Quiroz, an 11-year-old from Pharr, began drawing a slogan on red paper.

Roxanne said she wanted the migrant children in detention to know "they will be together with their families soon."

Her note was more defiant, though. It read: "Together, we will fight the battle."

The Associated Press contributed to this report.