OTAY MESA, UNITED STATES (AFP) - "Where are the children?" several women shout together from behind the detention centre wall. "We want the children free!"

"Do you hear us?" supporters ask from outside. "What do you need?"

This was the short interplay on Saturday between hundreds of protesters and people being held at a US detention centre for undocumented migrants in Otay Mesa, on the border with Mexico.

A group of people held at the centre had been separated from their children as part of President Donald Trump's "zero tolerance" policy.

The president on Wednesday signed an order ending the practice of splitting migrant families, but 2,300 children have already been separated from their parents.

"Shame," "Abolish ICE (Immigration and Customs Enforcement)," and "You're not alone," shouted the estimated 500 protesters rallied by religious leaders.

"I know those families are suffering and it is very hard to see that children have to go through the trauma of a detention centre when they are 10, five years old," Erica Leyva, 24, who travelled from Los Angeles to join the protest, told AFP.

Though she was born in the United States, she was briefly detained with her undocumented parents when she was four years old.

"I remember how it felt to be behind bars," she said.

Otay Mesa detention centre - managed by a private company - has the capacity for some 1,500 people, though in January it announced an expansion of 30 percent, according to local media.

More than 500 children have already been reunited with family members, according to a fact sheet released by the Department of Homeland Security on Saturday.

"The United States government knows the location of all children in its custody and is working to reunite them with their families," the department said.

How quickly the rest of the reunifications can be carried out remains a major question.

Lawyers working to bring families back together said they were struggling through a labyrinthine process, while more migrants continue to arrive.

Ever Sierra, deported after trying to enter the US, told AFP he planned to try again in a few days.

He arrived back in Honduras with his eight-month-old daughter's shoes hanging from his backpack.

She was being held in a detention centre in McAllen, Texas, along with her mother.

Benjamin Raymundo, a 33-year-old who was deported back to Guatemala, told AFP he left his home country in April with his five-year-old son Roberto, but the pair were separated when they were stopped by immigration officers in California.

A brother-in-law who lives in the US and a lawyer managed to find the child's whereabouts and the boy was eventually placed in this relative's custody.

"It's a great sadness for me, as if I'll never see my son again," he lamented.

Raymundo said he has no plans for now to return to the US. He hopes his son will be granted asylum.

On Friday, Democratic Senator Kamala Harris visited several undocumented women at the centre who were separated from their children.

"The stories they shared with me paint a picture of human rights abuses being committed by our government. We are so much better this," she said.

Demonstrators covered the name of the detention centre with a white sheet that read "concentration camp" in black letters and placed toys and stuffed animals around in protest.

Pastor and activist Ben McBride requested a prayer for "our immigrant relatives" before moving with a group to the fence, despite a "private property" sign and the warning of two guards armed with pepper spray.

At the gate, topped with barbed wire, they tied children's shoes and keys to demand the release and reunification with family of those detained.

"Release them!" they shouted. "Without justice there is no peace!"

"This is indecent, it is immoral," the pastor exclaimed to officials.

Apolonia Gregorio Jeronimo, 33, and two of her three children were visiting her detained husband at the centre.

Her was arrested six months ago and can be deported in August.

"I thank you very much for your support," said the woman, who told how her husband arrived in the US fleeing gang violence in Guatemala.

"He is desperate; he is sure that they kill him," explained Apolonia, who is a beneficiary of DACA, the programme that protects undocumented immigrants who arrived in the US as children.

Earlier in San Diego, 40km from the detention centre, some 1,500 people also protested against Trump's immigration policy.

"Families must be together", "No to the wall," and "Seeking asylum is still legal" were among the slogans on signs they carried.

And in response to the jacket First Lady Melania Trump wore on a trip to Texas to visit a detention centre for undocumented children - which bore the text "I really don't care. Do U?" - many people wore clothes with the opposite message: "I really care, do you?"