The President repeatedly threatened to close the US-Mexico border as the group of migrants, fleeing poverty and violence in their homelands, advance.
He also warned he would cut off aid to three countries if the group progressed.
In a series of tweets he said: “Sadly, it looks like Mexico’s police and military are unable to stop the caravan heading to the southern border of the United States.
“I have alerted Border Patrol and military that this is a national emergency.
“Guatemala, Honduras and El Salvador were not able to do the job of stopping people from leaving their country and coming illegally to the US.
“We will now begin cutting off, or substantially reducing, the massive foreign aid routinely given to them.”
Over the weekend the march was halted by Mexican police wearing riot gear at a border bridge with Guatemala.
Undeterred, some migrants swam or took a raft across the river to continue their quest.
Britany Hernandez said: “We have sunburn. We have blisters. But we got here. Our strength is greater than Trump’s threats.”
One Honduran mother of two said: “We want to give our children a better future.”