The annual caravan normally sets off during Holy Week.
A large group such as this one is harder to target and therefore offers more protection.
He's promised to deploy the military to close the southern border if the caravan is not stopped along the way.
They have since crossed into neighbouring Guatemala and then Mexico, with thousands more people joining along the way.
People line up to cross into the United States to begin the process of applying for asylum near Mexico.
"We feel like he's not human", said Carlos Fernandez, a 39-year old bricklayer, speaking by phone from the Guatemala-Mexico border after traveling since last Friday from the crime-wracked city of San Pedro Sula in Honduras.
When an estimated 4,000 to 5,000 migrants walked into Mapastepec on Wednesday, the municipality of 45,000 residents was ready.
The larger caravan now making its way north numbers in the thousands. "We just can't let them come in", the President told CBS 11 Political Reporter Jack Fink.
The question of how the migrant caravan began has wound its way to the American midterm elections.
"Everyone wants to know who is guilty, who is behind this", said Irineo Mujica, director of Tijuana-based Pueblos Sin Fronteras, which has advocated for this and previous caravans, helping to arrange the routes and other logistics.
Such destitution lies behind a caravan expected to leave San Salvador on October 31, said one of its organizers, Alex, who declined to give his last name for fear of reprisals. The stated goal of many is to get away from the violence in their home country and/or to find a job. The American people want a reasonable solution to our broken immigration system, and we are exhausted of lawlessness. It comes as a caravan of several thousand migrants makes it way north through Mexico. "We would do anything, except bad things".
Associated Press journalists traveling with the caravan have met throngs of Hondurans, as well as Guatemalans, Salvadorans and Nicaraguans, but no one from the Mideast.
Trump sought to persuade Mexican officials to stop the caravan for the past two weeks, as it has steadily grown after launching from Honduras, by threatening to reduce or end US aid.
How have other governments reacted?
"We ask Trump if he could help us because we come from a country that we had to leave because of its unemployment - we need to work".
"I'm not letting them in", he said during a bill signing in the Oval Office.
But more than 50 Honduran migrants interviewed by Reuters in recent days said they were anxious less about what would happen to them in the United States than the need to flee violence, unemployment and corruption at home.
There is no evidence that "unknown Middle Easterners" have infiltrated the migrant caravan heading toward the United States' southern border.
The migrants, many of whom are trudging north on worn shoes or bare feet, with some fainting from dehydration along the way say they are fleeing violence and murderous gangs in their homelands.
It is not illegal for anyone to seek asylum in another country. Additionally, another 116 people have agreed to return to their home countries and have been temporarily placed in the sites for asylum seekers.
The principles of the Protocol, and thereby the Convention, were integrated into the U.S. Refugee Act of 1980.
U.S. law enforcement officials arrested nearly 17,00 people trying to cross the southern border in September, up 31 percent from August.
Pentagon spokesman Navy Captain Bill Speaks said the Defense Department was now working with the Department of Homeland Security "to determine the specifics of our support" to border authorities. Their journey to the US-Mexican border is expected to take them weeks. The process can take months or even years.
Under current immigration law, for example, all individuals are entitled to certain rights and legal protections while in the custody of the US government, including illegal aliens.