Transcript for Thousands of Central American migrants have illegally crossed into Mexico

And we begin tonight with the growing humanitarian crisis and the political reverberations at home. Thousands on the move in Mexico, the crowds swelling in numbers after illegally pushing across the border in Guatemala. With hopes of eventually arriving here in the united States. The images underscoring how perilous the trek can be. Some parents carrying their children, crossing into Mexico by raft. And there have already been clashes as migrants try to escape violence in their home countries. And here at home, president trump threatening to use the military if necessary. And with just two weeks to the midterms, he's putting the blame squarely on Democrats. We begin with Matt Gutman, embedded in southwest Mexico. Reporter: A river of humanity is surging northward tonight. By the thousands, footsore Central American migrants have crossed into Mexico, heading towards the U.S. Hand in hand, they struck out from the town of ciudad Hidalgo at dawn, where so many illegally crossed this river into the Mexican country. Twice today, Mexican riot police deployed in force. They've got tear gas canisters, batons, full riot gear, and obviously fire extinguishers. They appeared ready for a reprisal of Friday's clashes. When thousands crammed the bridge between Guatemala and Mexico, thousands crammed the Gates. They were met with tear gas. Frantic parents shielding children. Some migrants leaping into the river and swimming across. An unknown number were injured. Today Mexican authorities offering asylum to control the flow. The vast majority of migrants turning it down, for now. Determined to reach America. But president trump vowing if Mexican authorities don't stop them, he will. If that doesn't work out, we're calling up the military, not the guard. We're calling up the military. Reporter: The mood festive at first, but soon turning desperate. Thirsty migrants swarmed locals. Local drivers taking pity, and hundreds crushing into cars, pickups, and trucks. Some hanging precariously off the sides. Anything to spell them from the 90-degree heat and the miles ahead. We met Blanca, gripping her children's hands. She's saying the father of these children was killed by gangs. And that's one of the reasons she's fleeing. Nothing, she says, is worse than going back to Honduras. A motorcyclist stopping to offer Blanca's children a ride. Another girl onboard suffering heatstroke. Her mother trying desperately to revive her. Delirious, the girl tried to get out, collapsing into my arms. There was no ambulance in sight. So, carefully, we folded her back into the rickshaw. For most of these migrants, the March started 500 miles away in Honduras, and tonight, they're still about 2,000 miles from the U.S. Border. Tom, everyone you see here has rejected asylum. It was offered multiple times by the Mexican government. They fear it's a ruse to deport them back to Honduras. They say they're going to continue to March northward to the United States, which means there will likely be a confrontation with Mexican authorities. Tom?

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