Hundreds of thousands of people have joined marches in towns and cities across the US to demand Donald Trump’s administration reunites families separated at the US-Mexico border.

More than 700 protests are taking place, with marchers moved by accounts of children separated from their parents dressed in white and carrying signs reading “No more children in cages”.

Protesters flooded streets in immigrant-friendly cities like New York and Los Angeles to conservative Appalachia and Wyoming.

They gathered on the front lawn of a Border Patrol station in McAllen, Texas, near a detention centre where migrant children were being held in cages, and on a street corner near Mr Trump’s golf resort at Bedminster, New Jersey, where the president is spending the weekend.

Immigration protestProtesters chant slogans as they march in New York (AP Photo/Mary Altaffer)

Mr Trump has backed away from the family separation policy amid bipartisan and international uproar, and those marching on Saturday demanded the government quickly reunites the families that were already divided.

In the president’s hometown of New York City, an estimated 30,000 marchers poured across the Brooklyn Bridge in sweltering heat, some carrying their children on their shoulders, chanting “Shame!”

The families split up as they tried to enter the U.S. illegally were largely fleeing extreme violence, persecution or economic collapse in their home countries, often in Central America.

In Washington, another massive crowd gathered in Lafayette Park across from the White House in what was expected to be the largest protest of the day, stretching for hours under a searing sun. Firefighters at one point misted the crowd to help people cool off.

Lin-Manuel Miranda, creator of the musical Hamilton, sang a lullaby dedicated to parents unable to sing to their children. Singer-songwriter Alicia Keys read a letter written by a woman whose child had been taken away from her at the border.

Smaller groups came together in city parks and squares in every state, a total of 703 places across the country, and photos quickly started ricocheting around social media.

Children joined in, with one little girl in Washington carrying a handwritten sign which said: “I get my mommy. Why can’t she?”

In Portland, Oregon, several stay-at-home mothers have organised their first rally while caring for young children.

“I’m not a radical, and I’m not an activist,” said Kate Sharaf, a Portland co-organiser. “I just reached a point where I felt I had to do more.”

Immigrant advocacy groups say they are thrilled – and surprised – to see the issue gaining traction among those not tied to immigration.

“Honestly, I am blown away. I have literally never seen Americans show up for immigrants like this,” said Jess Morales Rocketto, political director at the National Domestic Workers Alliance.

“We just kept hearing over and over again, if it was my child, I would want someone to do something.”

Tyler Houlton, a spokesman for the US Department of Homeland Security, welcomed interest in the immigration system and said only Congress has the power to change the law.

Immigration protestOne protest began in Lafayette Square across from the White House, (AP Photo/Alex Brandon)

“We appreciate that these individuals have expressed an interest in and concern with the critical issue of securing our nation’s borders and enforcing our immigration laws,” he said.

“As we have indicated before, the department is disappointed and frustrated by our nation’s disastrous immigration laws and supports action.”

Mr Trump took to Twitter on Saturday morning to show his support for Immigration and Customs Enforcement (ICE) amid calls from some Democrats for major changes to immigration enforcement.

Tweeting from New Jersey, Mr Trump said Democrats “are making a strong push to abolish ICE, one of the smartest, toughest and most spirited law enforcement groups of men and women that I have ever seen”.

He urged ICE agents to “not worry or lose your spirit”.