Armed vigilantes are illegally detaining hundreds of migrants attempting to cross the southern US border, human rights activists have claimed.

Ex-military veterans dressed in camouflage and armed with semi-automatic rifles can be seen rounding up Central Americans on videos shared online.

The American Civil Liberties Union (ACLU) claimed the Trump administration’s “vile racism has emboldened white nationalists and fascists” to act outside the law.

The United Constitutional Patriots (UCP) has patrolled a stretch of the border in Sunland Park, New Mexico, since late February, regularly uploading footage of its activities on Facebook.

In one video, thought to have been shot on Tuesday evening, UCP members are seen detaining hundreds of migrants. Many appeared to be families with children, who were then held until border patrol agents arrived.

“This group was so huge we wont [sic] have an accurate count until the [Border Patrol] is finished processing them,” the group wrote. “This needs to stop!!!!! Build the wall.”

Trump prototypes for Mexico border wall demolished in California Show all 20 1 /20 Trump prototypes for Mexico border wall demolished in California Trump prototypes for Mexico border wall demolished in California The prototypes for President Trump’s border wall are being demolished. AP Trump prototypes for Mexico border wall demolished in California The US Customs and Border protection had built the eight 30-foot tall steel and concrete models near San Diego on the US-Mexico border. Reuters Trump prototypes for Mexico border wall demolished in California A 2018 report from the Government Accountability Office found that the construction challenges presented by the four concrete models would be “extensive” and those presented by two of the other models would be “substantial”. Reuters Trump prototypes for Mexico border wall demolished in California It was intended that Mr Trump would choose his favourite of the designs after testing had been completed. AFP/Getty Trump prototypes for Mexico border wall demolished in California The President is yet to comment. AFP/Getty Trump prototypes for Mexico border wall demolished in California Each prototype cost $300,000 to $500,000 to build and they are being knocked down in order to make way for the San Diego Secondary Wall project which will see up to 14 miles of barrier being built to support the existing steel border fence. AP Trump prototypes for Mexico border wall demolished in California The new barrier will not employ the design of any of the prototypes, instead being built of the favoured steel bollards which make up the current section of the wall at San Diego. Reuters Trump prototypes for Mexico border wall demolished in California The new secondary barrier being built near San Diego. Reuters Trump prototypes for Mexico border wall demolished in California The rubble of one of the demolished prototypes. Reuters Trump prototypes for Mexico border wall demolished in California A steel wall prototype AFP/Getty Trump prototypes for Mexico border wall demolished in California A wall prototype made from a mix of steel and concrete AFP/Getty Trump prototypes for Mexico border wall demolished in California A wall prototype made from a mix of steel and concrete AFP/Getty Trump prototypes for Mexico border wall demolished in California A concrete wall prototype AFP/Getty Trump prototypes for Mexico border wall demolished in California A steel wall prototype AFP/Getty Trump prototypes for Mexico border wall demolished in California A concrete wall prototype AFP/Getty Trump prototypes for Mexico border wall demolished in California A concrete wall prototype AFP/Getty Trump prototypes for Mexico border wall demolished in California A concrete wall prototype AFP/Getty Trump prototypes for Mexico border wall demolished in California A digger approaches the prototype wall AFP/Getty Trump prototypes for Mexico border wall demolished in California Aerial view of the wall prototypes at the US-Mexico border after they were torn down AFP/Getty Trump prototypes for Mexico border wall demolished in California An aerial view showing Tijuana, Mexico on the left and the demolished wall prototypes on the right AFP/Getty

Dozens more videos show similar detentions of the migrants – an increasing number of whom are seeking asylum in the US after fleeing gang violence in their home countries.

The organisation says its members “stand up for the rights of every American” and “help keep America safe”.

However, the vigilante detentions on the border have been condemned by the ACLU, which described UCP as a “fascist militia organisation”.

In a letter to New Mexico governor Michelle Lujan Grisham and state attorney general Hector Balderas, the ACLU alleged the detentions were illegal and called for an investigation into the situation.

It blamed Donald Trump anti-migrant rhetoric for encouraging the vigilante groups.

“[The United Constitutional Patriots’] actions undermine the legitimate efforts of our state’s law enforcement officials to keep New Mexico families safe and they erode community trust,” the letter added.

“The Trump administration’s vile racism has emboldened white nationalists and fascists to flagrantly violate the law.

“This has no place in our state: we cannot allow racist and armed vigilantes to kidnap and detain people seeking asylum. We urge you to immediately investigate this atrocious and unlawful conduct.”

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The offices of Ms Lujan Grisham and Mr Balderas did not respond to requests for comment.

In March, then-US Customs and Border Protection commissioner Kevin McAleenan said authorities did not need the help of citizens to police the border.

“We are not asking for civil society groups to provide border security assistance,” he said during a visit to Texan city El Paso.