US President Donald Trump has said he has alerted the Border Patrol and military that the caravan of people heading to the US/Mexico border is a national emergency.
Mr Trump made the comment in a tweet saying the group includes "Criminals and unknown Middle Easterners".
He went on to tweet that illegal immigration in the US is due to Democrats "not giving us the votes to change our pathetic immigration laws!"
Thousands of mostly Honduran migrants have been heading towards the US border with Mexico despite President Trump declaring that they are unwelcome.
Sadly, it looks like Mexico’s Police and Military are unable to stop the Caravan heading to the Southern Border of the United States. Criminals and unknown Middle Easterners are mixed in. I have alerted Border Patrol and Military that this is a National Emergy. Must change laws!— Donald J. Trump (@realDonaldTrump) October 22, 2018
He asked people to "Remember the Midterms!", referring to the elections on 6 November where 35 Senate seats are up for grabs along with the 435 seats in the House of Representatives.
The Republicans currently have control of both chambers of Congress.
Mr Trump went on to say that as Guatemala, Honduras and El Salvador were not able to stop people from leaving their country and coming illegally to the US, "massive foreign aid routinely given to them" will now be cut off, or substantially reduced.
Members of the caravan, exhausted from the hours-long trek on foot from the Guatemalan border, mostly ignored police offers in Mexico to board buses heading to a migrant shelter because of suspicions they might be deported instead.
The migrants have defied threats by Mr Trump that he will close the US-Mexico border if the caravan advances, as well as warnings from the Mexican government that they risk deportation if they cannot justify seeking asylum in Mexico.
In southern Mexico, police in riot gear shadowed the caravan's arrival along a southern highway, but did not impede their journey.
Most said they felt safer advancing in a large group.
But some of the migrants have already decided to return to their homes. At the Honduran border town of Corinto, immigration officials checked documents of migrants who then boarded buses to return to their homes.
More than 3,400 Hondurans migrants were returned to their home country over the weekend, according to a migrant assistance body.
On Saturday Mexican authorities attempted to resolve the challenge posed by the caravan waiting to enter Mexico's southern border by legal means.
After stopping a group of migrants who tried to enter Mexico on Friday, Mexican authorities said they would willingly permit migrants to enter the country if they had the required documents and visas or were applying for asylum.
On Friday, President Enrique Nieto said in a message on national television that Mexico does not permit nor will it permit irregular entry "much less for those who attempt to do so violently, but he did offer help for those who wish to enter "respecting the laws".
Some 2,000 migrants have already crossed the border into Mexico and are camping out while completing their migration procedures. At least 1,000 more remain at the border, waiting to enter the country, reported the EFE Agency.
Temperatures climbed to 29C over the weekend. Many migrants, especially children, are suffering in the hot and humid weather, as they look for sources of food and water.
Hundreds of Honduran migrants decided on Saturday to cross the Suchiate River in their eagerness to reach the US.
Mexican immigration officials approached caravan migrants earlier to propose an orderly entry plan and set out to solve related problems. Meanwhile, Mexico's National Human Rights Commission is working on preventing possible violations.
Additional reporting: Reuters