Homeland Security rushed to barricade part of the country’s busiest land border crossing Monday after learning that some of the thousands of migrants massed on the Mexican side were planning to attempt a kamikaze run to sneak in.

As many as 10,500 migrants are currently camped in Tijuana and elsewhere in Mexico, officials said, most of them part of the caravans of Central Americans that broke into Mexico last month, overwhelming authorities on that country’s southern border.

Intent on avoiding those chaotic scenes here, U.S. Customs and Border Protection shut down northbound lanes of traffic at the San Ysidro Port of Entry between Tijuana and San Diego for three hours early Monday, using that time to add more razor wire and barricades they hope would cut down on a mass incursion.

The maximum 26 entry lanes for vehicles have also been reduced to 15, meaning the caravans are now threatening legal commerce and cross-border visitors, officials said.

“Unfortunately, some members of the caravan are purposely causing disruptions at our border ports of entry,” Homeland Security Secretary Kirstjen Nielsen said on Twitter as she described the new security measures. “There is a legal and illegal way to enter the U.S. We have deployed additional forces to protect our border. We will enforce all our laws.”

U.S. officials said they’d received intelligence that caravan members were massing overnight and plotting an attempt to overrun the San Ysidro and Otay Mesa ports of entry, including a possible effort to have large numbers of people run through the traffic lanes.

Both the Mexican and U.S. governments took steps to try to prevent such a surge.

“We made the determination that we needed to add more control measures at the port of San Ysidro,” a senior Homeland Security official said.

He said the northbound lanes were shut down from 3:15 a.m. to 6:25 a.m., and when they reopened they were down to just 15 lanes of traffic.

Asked whether hardening the entry points would push migrants to make more dangerous crossings between the ports, jumping the border fence or going through desert, another official said the U.S. wouldn’t allow itself to be trapped by migrants’ decisions.

“We are not going to make it easier to illegally enter the United States,” that official said.

Mr. Trump last month ordered a massive troop deployment to the border ahead of the caravan — and ahead of the midterm congressional elections.

But now it appears they’ll start coming home. Politico reported that some have already completed their missions and will return home as early as this week. Currently there are 5,800 troops deployed: 1,500 in Arizona, 1,500 in California and 2,800 in Texas.

Army Lt. Gen. Jeffrey Buchanan told Politico he expects the troops will be packed up by Dec. 15.

They were sent to “harden” the border, adding razor wire and barriers to places that needed a quick plus-up, Homeland Security officials said.

Some troops flew Border Patrol agents into the field as quick-reaction forces when illegal incursions are detected. But the troops were not engaged in actual enforcement.

The Pentagon has refused to disclose the cost of the deployment.

Immigrant-rights advocates Monday denounced the troop commitment and call for a speedy withdrawal.

Ned Price, a former official on President Obama’s National Security Council, pointed to Mr. Trump’s relentless commenting on the caravan ahead of the election, compared with the far lesser amount of presidential oxygen expended after the voting was done.

“It is clear that the deployment was needlessly done to score political points,” he said. “Migrants at the border can be processed orderly and safely as asylum seekers — it’s what we’ve always done. These caravans do not pose the risks that the president claims they do.”

Activists have challenged a new administration policy that blocks illegal immigrants who jump the southwest border from attempting to claim asylum. Those who queue to come through the ports of entry are still permitted to make claims.

The goal is to funnel traffic into legal channels.

The American Civil Liberties Union has sued over the policy, and a court in California held a hearing on the matter Monday.

Mexico, Honduras, El Salvador and Guatemala are also challenging the policy, with their human rights ombudsmen filing a complaint with the Inter-American Commission on Human Rights over the policy, arguing their citizens should be allowed to flee their countries to find refuge in the U.S.

The complaint, reported by El Universal, a Mexican newspaper, says migrants who make it into the U.S., even if entering illegally, must be allowed to demand asylum in the U.S.

While the Mexican government has taken an accommodating stance toward the caravan, some of its residents are fed up.

El Mexicano, a Tijuana newspaper, reported on protests that erupted complaining about the entitled stance of some caravan migrants.

The demonstrators sang the “Himno Nacional Mexicano,” or national anthem, and chanted “Tijuana,” as they demanded immediate deportation of the Central American migrants using their city as a camp and staging point.

Among the gripes, according to El Mexicano, was the caravan’s complaints over food they’ve been given at shelters, and demands that they do small chores in exchange for a place to stay.

⦁ Ben Wolfgang contributed to this article.

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