A Californian couple has raised more than $5m (£3.8m) in three days to help reunite immigrant families who have been separated at the US border, after an outpouring of support from Facebook users.

Charlotte and Dave Willner set up the fundraiser after they saw a viral photo of an anguished two-year-old Honduran girl looking up at her mother, who was being searched by a US Border Patrol agent in southern Texas.

The couple have a two-year-old daughter of their own, and the image made them want to help the families being separated under the Trump administration’s “zero-tolerance” immigration policy.

On Saturday morning, they started a Facebook fundraising page for the Refugee and Immigrant Centre for Education and Legal Services (RAICES), a nonprofit organisation which provides low-cost legal defence services to immigrant and refugee families in Texas.

The Willners set a modest goal of $1,500 (£1,100).

Three days later, it has garnered more than $5m (£3.8m) from more than 130,000 people, stunning the staff at RAICES.

“We’ve had moments of ecstasy and there have been a lot of tears in response to this outpouring of support,” Jonathan Ryan, executive director of RAICES, said in a phone interview. “But those moments of joy are curtailed by a realisation of great responsibility.”

The funds will go towards legal representation for immigrant children and parents in Texas, as well as towards paying parents’ bond so they can be released from detention centres and reunited with their children.

The organisation — which has about 50 lawyers on staff — plans to go on a hiring spree and fund training for volunteer lawyers willing to travel to Texas to assist, Mr Ryan said.

Undocumented immigration across the US-Mexico border Show all 14 1 /14 Undocumented immigration across the US-Mexico border Undocumented immigration across the US-Mexico border Immigrant children, many of whom are separated form their parents, are housed in Texas' tent city Reuters Undocumented immigration across the US-Mexico border A two-year-old Honduran asylum seeker cries as her mother is searched and detained near the US-Mexico border Getty Undocumented immigration across the US-Mexico border Undocumented migrants ride on the top of a freight train referred to as the beast, or La Bestia Getty Undocumented immigration across the US-Mexico border A cage inside a US Customs and Border Protection detention facility in Texas Reuters Undocumented immigration across the US-Mexico border US Border Patrol Academy All new agents must complete a months-long training course at the New Mexico facility before assuming their posts at Border Patrol stations, mostly along the US-Mexico border Getty Undocumented immigration across the US-Mexico border US-Mexico border fence A group of young men walk along the Mexican side of the US-Mexico border fence in a remote area of the Sonoran Desert Getty Undocumented immigration across the US-Mexico border US-Mexico border fence in the US Man looks through US-Mexico border fence into the US in Tijuana, Mexico Getty Undocumented immigration across the US-Mexico border US-Mexico border fence US Border Patrol agent Sal De Leon stands near a section of the US-Mexico border fence while stopping on patrol on in La Joya, Texas Getty Undocumented immigration across the US-Mexico border US Border Patrol Academy US Border Patrol instructor yells at trainees after their initial arrival to the academy Getty Undocumented immigration across the US-Mexico border Memorial service in Guatemala Families attend a memorial service for two boys who were kidnapped and killed in San Juan Sacatepequez, Guatemala. Crime drives emigration from Guatemala to the United States, as families seek refuge from the danger Getty Undocumented immigration across the US-Mexico border Arrests on the border Undocumented immigrants comfort each other after being caught by Border Patrol agents near the US-Mexico border Getty Undocumented immigration across the US-Mexico border Detention holding facility A boy from Honduras watches a movie at a detention facility run by the US Border Patrol Getty Undocumented immigration across the US-Mexico border Mexican farm workers Mexican migrant workers harvest organic parsley at Grant Family Farms in Wellington, Colorado Getty Undocumented immigration across the US-Mexico border Mexican family in Arizona A Mexican immigrant family sits in the living room of their rented home in Tuscon, Arizona. The family that Arizona's new tough immigrant law had created a climate of fear in the immigrant community. Getty

The Trump administration policy, which aims to criminally prosecute all immigrants crossing the border illegally, has resulted in nearly 2,000 children being taken away from their parents in six weeks.

Public outcry over family separations at the border has risen in recent days as children’s experiences in custody have been documented.

A recording of children calling out desperately for their parents after being separated from them was released Monday by ProPublica and was met with immediate outrage.

Donald Trump has defended the policy by saying that people crossing the border “could be murderers and thieves and so much else.”

Kirstjen Nielsen, the secretary of Homeland Security, said in a news briefing on Monday the only way the practice would change is through legislation in Congress, even though there is no law requiring that families be separated at the border.

On Tuesday afternoon, the Facebook page — called “Reunite an immigrant parent with their child” — was receiving about $3,000 per minute, according to a spokeswoman for the Willners, who are both former Facebook employees.

“When we look at the faces of these children, we can’t help but see our own children’s faces,” Ms Willner told The Mercury News of San Jose, California.

The Willners’ page is the largest single fundraiser in Facebook’s history, said Roya Winner, a Facebook spokeswoman.

Several broader campaigns made up of multiple fundraisers, like one created in the aftermath of Hurricane Harvey, have raised more.

Other single fundraisers which have recently been highly successful include a Norwegian campaign for Doctors Without Borders, which garnered $2 million, and an effort to raise money for medical aid to Palestinians in Gaza, which raised $1.7m (£1.3m).

Photos of humanitarian crises have galvanised the public for decades, but internet fundraising platforms have enabled donations to stream in.

In 2015, images of a 3-year-old Syrian Kurdish boy whose body had washed up on a beach in Turkey caused a spike in donations to humanitarian efforts addressing the Syrian civil war.

The average number of daily donations to the Red Cross relief effort in Syria increased 100-fold in the week after the images were published compared with the week before.

Mr Ryan said he did not yet know when the millions of dollars in donations would reach RAICES, but said it is already starting to deploy its lawyers to assist detained immigrants and their children.

Costs for this kind of legal assistance can add up quickly, he said.

Bond for a single detained immigrant can be set at $10,000 (£7,600).

Lawyers must locate the parents, travel to detention centres and meticulously prepare applications for asylum.

“These people need lawyers,” Mr Ryan said. “Beyond the punditry and politics, each person suffering from this policy has a huge legal case to begin to prepare for.”