US President Donald Trump has ordered National Guard personnel to the southern US border in an attempt to clamp down on illegal immigration from Mexico.
Mr Trump has ratcheted up the pressure on both the US Congress and Mexico in recent days to take action to stem illegal immigration.
The president has been infuriated by reports of a "caravan" of Central American migrants trekking towards the US border.
Mexican officials have screened people in the caravan, stalling it and dispersing migrants.
The Mexican foreign ministry denied putting pressure on the migrants, saying that the caravan was breaking up of its own volition.
The US-based advocacy group that organises the annual event, Pueblo Sin Fronteras, has said it will now end in Mexico City, rather than at the US border as had been planned.
Some migrants may get permits to stay, while others will be sent back to their home countries, the Mexican government said.
Organisers say the event is designed to raise awareness about the plight of migrants rather than reach the United States, but some participants have travelled to the border in the past.
Mr Trump took a hard line on illegal immigration during the 2016 US presidential election campaign and is frustrated by the failure of Congress to fund his long-promised border wall.
His order to send National Guard troops to the frontier with Mexico came after data showed that illegal immigration has sharply rebounded following a plunge in his first year in office.
Mr Trump's warning of a crackdown when he entered the White House in January 2017 drove the number of apprehensions of illegal border crossers - an indicator of total crossing numbers - to a four-decade lows.
"We continue to see unacceptable levels of illegal drugs, dangerous gang activity, transnational criminal organisations and illegal immigration flow across our southern border," Secretary of Homeland Security Kirstjen Nielsen told reporters at the White House yesterday.
"This threatens not only the safety of our communities and children but also our rule of law. It's time to act.
"The Department of Defense and the Department of Homeland Security have been directed to work together with our governors to deploy the National Guard to our southwest border, to assist the border patrol.
"We do hope that the deployment begins immediately. Today is the day we want to start this process. The threat is real."
The US National Guard has previously been deployed to help patrol the southern border, in 2010 under former president Barack Obama, and from 2006-2008 under George W Bush.
