NEW YORK (NYTIMES) - It all started with a viral photo of a toddler crying as her mother was detained at the border.

Charlotte and Dave Willner saw it on the internet, like so many other people, and responded by starting a fundraising page that would rapidly become the largest single fundraiser in Facebook's history.

The Bay Area couple had been struck by the sight of the anguished two-year-old Honduran girl looking up at her mother, who was being searched by a US Border Patrol agent in southern Texas. They 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 (June 16) morning, they started a Facebook fundraising page for the Refugee and Immigrant Centre for Education and Legal Services, or RAICES, a non-profit organisation that provides low-cost legal defence services to immigrant and refugee families in Texas. The Willners set a modest goal of US$1,500 (S$2,034).

Three days later, it has garnered more than US$5 million 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 toward legal representation for immigrant children and parents in Texas, as well as toward 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, Ryan said.

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.

President Donald Trump has defended the policy by saying that people crossing the border "could be murderers and thieves and so much else." And Kirstjen Nielsen, the secretary of Homeland Security, said in a news briefing on Monday that 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 page - called "Reunite an immigrant parent with their child" - was receiving about US$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," Willner told The Mercury News of San Jose, California. Charlotte Willner now works at Pinterest and Dave Willner at Airbnb, according to a spokesman for the couple.

The Willners' page is the largest single fundraiser in Facebook's history, said Roya Winner, a Facebook spokesman. Several broader campaigns made up of multiple fundraisers, like one created in the aftermath of Hurricane Harvey, have raised more. Other single fundraisers that have recently been highly successful include a Norwegian campaign for Doctors Without Borders, which garnered US$2 million, and an effort to raise money for medical aid to Palestinians in Gaza, which raised US$1.7 million.

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.

Ryan said that he did not yet know when the millions of dollars in donations would reach RAICES, but that 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 US$10,000. Lawyers must locate the parents, travel to detention centres and meticulously prepare applications for asylum.

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