
The U.S. government has begun 'hardening' the border crossing from Tijuana, Mexico, to prepare for the arrival of a migrant caravan trekking inexorably toward the border.

Hardening operations began on Tuesday at the San Ysidro port of entry in California, where a spear-tip group of 400 migrants from the first caravan arrived the same day.

Citing family ties and more generous benefits in California, the main caravan tacked westward after departing Mexico City, opting to add several hundred miles to their trek rather than attempt a crossing into Texas. The main body of some 7,000 is now in Jalisco state, about 1,300 miles from Tijuana.

The U.S. responded by rushing troops to California, closing four lanes at the busy San Ysidro and Otay Mesa ports of entry in San Diego to lay barbed wire.

The main body of the first migrant caravan of 9,000 walks along the road between Zapopan and Tequila in the Mexican state of Jalisco, on their trek north, on Tuesday. They are now 1,300 miles away and averaging 185 miles a day by hitchhiking

A Marine installs barbed wire barriers requested by CBP at the San Ysidro port of entry in San Diego on Tuesday

U.S. Marines work to move building materials as they harden the border with Mexico in preparation for the arrival of a caravan of migrants at the San Ysidro border crossing in San Diego on Tuesday

Marines help to build a concertina wire barricade at the U.S. Mexico border at the San Ysidro border crossing on Tuesday

Department of Defense personnel installs barriers requested by Custom and Border Protection at the San Ysidro port of entry

Marines installs barriers at the San Ysidro port of entry, anticipating the arrival of Central American migrants heading towards the border, as seen from the Mexican side of the border in Tijuana, Mexico, on Tuesday

This map shows the position of the main caravan as of Tuesday. The migrants are moving toward the border at Tijuana

Customs and Border Protection said the closures were needed 'to install and pre-position port hardening infrastructure equipment in preparation for the migrant caravan and the potential safety and security risk that it could cause.'

The temporary lane closures still leaves a substantial path for the tens of thousands of people who cross the border daily: Twenty-three lanes remain open at San Ysidro and 12 at Otay Mesa.

Some 400 migrants who broke away from the main caravan in Mexico City arrived in the border city of Tijuana on Tuesday by bus, according to a witness. Larger groups are expected to arrive in the coming days, human rights organizations said.

The first caravan migrants to arrive at the border were undeterred by the U.S. military presence.

'I prefer to be in detention in the United States than to return to my country, where I know they are going to kill me for being different,' said Nelvin Mejía, a transgender woman who arrived in Tijuana on Monday with a group of about 70 people seeking asylum. 'Last month, they killed my partner, and I do not want to end up like that.'

Migrants in the caravan get a free ride on a truck past agave farms as they depart Guadalajara, Mexico on Tuesday

The caravan is seen moving on the roads in the state of Jalisco, Mexico on Tuesday. Hitchhiking has sped their progress

Caravan migrants walk along the road between Zapopan and Tequila in the Mexican state of Jalisco on Tuesday

U.S. Defense Secretary Jim Mattis said he would travel to the border area on Wednesday, his first visit since the military announced that over 7,000 U.S. troops would go to the area as the caravan of mostly Hondurans has made its way through Mexico.

'CBP has been and will continue to prepare for the potential arrival of thousands of people migrating in a caravan heading towards the border of the United States,' Pete Flores, the agency's director of field operations in San Diego, said in a statement, citing a 'potential safety and security risk.'

The caravans became a campaign issue in midterm elections and President Donald Trump has ordered the deployment of over 5,000 military troops to the border to help fend off the migrants.

To the thousands of Central American migrants in the main caravan now making their way toward Mexico's Pacific coast state of Nayarit, the prospect of meeting a hostile reception at the border is nothing new.

After a month on the road, through three countries, migrants like Maribel, 22, from La Ceiba, Honduras, are used to tough conditions.

Maribel, who did not want her last name used for fear of reprisals in Honduras, pushed a baby carriage with her one-year-old daughter while her husband pushed another with their 3-year-old son along a highway leading from Guadalajara to the Nayarit capital of Tepic.

'We are well aware of everything Trump has been saying,' said Maribel. 'Let them close whatever they want to close, but we are going to get through anyway.'

Caravan migrants get ready to leave the Benito Juarez Auditorium they used as a shelter overnight in Guadalajara on Tuesday

Migrants in the caravan walk along the road connecting Guadalajara with Tepic, Mexico as they trek north on Tuesday

Caravan migrants continue their march to the United States on roads in the state of Jalisco, Mexico on Tuesday

The thousands of Central American migrants left shelters in Guadalajara early Tuesday and were taken by bus to a highway tollbooth to wait for rides to their next destination.

They thought other buses would be waiting for them to take them through hurricane-ravaged Nayarit to the neighboring state of Sinaloa, further north. But no other buses showed up and few trucks passed to pick them up, leaving many to walk.

Most appeared intent on taking the Pacific coast route northward to the border city of Tijuana, which was still about 1,350 miles away. The migrants have come about 1,500 miles since they started out in Honduras around October 13.

While they previously suffered from the heat on their journey through Honduras, Guatemala and southern Mexico, they now trek along highways wrapped in blankets to fend off the morning chill.

While the caravan previously averaged only about 30 miles a day, the migrants are now covering daily distances of 185 miles or more, partly because they are relying on hitchhiking rather than walking.

Migrants have hopped aboard different kinds of trucks, regardless of comfort or safety. Some have stacked themselves four levels high on a truck intended for pigs.

A migrant hitchhikes on the road between Zapopan and Tequila in the Mexican state of Jalisco on Tuesday

Members of the caravan of Central American migrants continue their march to the US on roads in the state of Jalisco, Mexico

Caravan migrants climb aboard a tanker truck in the Mexican state of Jalisco on Tuesday

A member the caravan tries to fit himself into a truck's spare wheel compartment, parked on the highway connecting Guadalajara and Tepic, Mexico, on Tuesday. Migrants thought buses would be waiting for them to take them through hurricane-ravaged Nayarit to the neighboring state of Sinaloa, further north, but no buses showed up and few trucks passed

On Monday, a few boarded a truck carrying a shipment of coffins, while others squeezed into a truck with narrow cages used for transporting chickens.

Many, especially men, travel on open platform trailers used to transport steel and cars or get in the freight containers of 18-wheelers and ride with one of the back doors open to provide air flow.

Last month, a Honduran man in the caravan died when he fell from a platform truck in the Mexican state of Chiapas.

A smaller, second caravan began arriving in Mexico City on Monday. By Tuesday, over 1,000 migrants had set up camp at the same Mexico City sports complex the larger caravan left Saturday. A third caravan was heading toward the capital.

Many say they are fleeing rampant poverty, gang violence and political instability primarily in the Central American countries of Honduras, Guatemala, El Salvador and Nicaragua.

Mexico has offered refuge, asylum or work visas, and its government said Monday that 2,697 temporary visas had been issued to individuals and families to cover them during the 45-day application process for more permanent status.

Some 533 migrants had requested a voluntary return to their countries, the government reported.

Second Caravan: In Mexico City, migrants carrying donated coffee and breakfast pass by a map of the metro system, which is waving fares for migrants, at a sports complex sheltering the second caravan of about 1,000 migrants on Tuesday

Second Caravan: Volunteers organize free breakfasts for migrants at a sports complex sheltering various Central American migrant caravans trying to reach the U.S. border, in Mexico City on Tuesday