CÚCUTA, Colombia — This Colombian border city, already grappling with a mounting migration crisis, is bracing for a tense showdown that is likely to mark a new chapter in the unfolding crisis of its neighbor, Venezuela.

U.S. military planes continue a buildup of humanitarian aid on the Venezuelan border, a development spotlighted Sunday in an appearance by Sen. Marco Rubio (R-Fla.). More than 100,000 people are expected at a foreign-
organized concert here on Friday to protest the Venezuelan government. And on Saturday, Venezuelan opposition figures with U.S. backing plan to lead thousands of protesters, dressed in white, to the border to confront the country’s national guard.

All of this has made Cúcuta, a city of 750,000 overwhelmed by the massive Venezuelan migration that has flowed over its border for years, very nervous.

Venezuelan President Nicolás Maduro has vowed to block the aid, calling it a pretext for U.S. intervention in his crisis-stricken country. Venezuela is in the midst of a dire humanitarian crisis, and the relatively small amount of aid is meant to begin to relieve a severe shortage of basic goods that has fueled malnutrition and disease. But opposition leaders and U.S. officials also hope to elicit the first public mass defection of Venezuelan armed forces if the soldiers disavow their orders and accept the shipment.

During a visit Sunday to Cúcuta, Rubio encouraged troops to defect, calling the Maduro government “a criminal regime willing to starve and kill its own people.”

“There comes a time in many people’s lives when they have to make a decision that will define them forever,” Rubio said. “That time has come for the Venezuelan soldiers.”

Other American officials have also been clear that they want Maduro out of power and that they see pushing aid into the country as one step toward achieving that outcome. Some major aid organizations have refused to participate, objecting to the use of humanitarian aid for political ends.

“Maduro has got to go,” Mark Green, head of the U.S. Agency for International Development, said at a news conference Saturday in Cúcuta while 80 tons of food and hygiene products were unloaded from a C-17 cargo plane behind him.

Officials say Cúcuta is the spearpoint of an operation that will breach Venezuela’s border with humanitarian aid from all sides, with other staging spots in Brazil and the island of Curacao.

But planners have not been able to say just how they will do that. On Sunday, Venezuelan opposition officials handed out fliers on the Simón Bolívar International Bridge, where thousands of desperate Venezuelans cross each day to buy food or to migrate elsewhere in South America.

Opposition leaders called on Venezuelans there to join the effort to move the aid over their border but couldn’t say exactly where or when.

“We still haven’t decided which border crossing to use,” said Alcides Monsalve, mayor of the Venezuelan city of Merida, who traveled to Cúcuta to help the effort. “We don’t have any idea.”

In a speech to volunteers in Caracas this weekend, Juan Guaidó — head of the opposition-controlled National Assembly who has evoked the constitution in claiming himself the nation’s true president — said more than 600,000 Venezuelans had agreed to join the massive effort planned for Saturday. He said volunteers would receive further directives on Monday via email.

“There will be mobilizations in all cities around the country on Feb. 23,” Guaidó told a cheering crowd on Saturday. “Not just caravans going to the border. It will be a whole country in the street.”

Leaders in Cúcuta insist that the Venezuelan soldiers guarding the border crossings will accept the food aid, motivated by the hunger of their own families. But Venezuelans living on the Colombian side of the border expressed doubt.

“They won’t let it pass,” said Victor Mora, a Venezuelan sitting near the border with the eight members of his salsa band, who all nodded in agreement. “The military structure has them really indoctrinated. They are scared to disobey.”

But many said they would join the effort to confront the border guards.

“If Venezuelans can walk 20 days to Peru, then of course we can walk across the bridge to accompany the aid,” said Julio Campos, 45, who has lived in Cúcuta for a month.

Four years ago, he said, he was member of his town council in the Venezuelan coastal city of Puerto La Cruz, with a family, an apartment, a 2010 Chevrolet Optra and his own business in wholesale cheese distribution. Then he watched the economy tank as food became scarce and his business folded.

Now, like thousands of others, he pays about $1 per night to sleep on the floor of a large room with dozens of other Venezuelans in Cúcuta, where he makes a small commission bringing bus passengers to travel agencies. He aims to send money home to his wife and three kids, but since arriving in early January, he has only been able to do so once.

He didn’t think the soldiers would let the aid pass but pledged to join the effort anyway.

“For us Venezuelans living in Colombia, it’s better to die without fear than to live crying,” he said.

Wilfredo Cañizares, an activist in Cúcuta and director of Fundación Progresar, which tracks organized crime in the border zone, said local leaders have felt powerless as the big week looms.

“It’s all being driven by the national government and the U.S.,” he said.

He also raised alarm about Friday’s concert, announced Thursday by British billionaire and airline magnate Richard Branson. The event, set to take place on a highway a little over a mile from the Venezuelan border, is intended to raise $100 million to address the humanitarian crisis there.

A spokesman for the Cúcuta police on Sunday said that local authorities have yet to meet with concert organizers.

“It seems supremely inconvenient to organize a concert at this moment,” Cañizares said. “We are very worried by everything.”

In Venezuela late Sunday, five members of the European Parliament invited by the opposition to meet with Guaidó in Caracas were barred from entering the country at Simón Bolívar International Airport, according to the opposition and members of the delegation. They were subsequently deported by Venezuelan authorities. 

“Attention, our passports have been taken and we’re being thrown out of Venezuela,” tweeted Spain’s Esteban González Pons. “We’re being mistreated and the only explanation we’re given is that Maduro doesn’t want us here.”

Venezuela's foreign minister, Jorge Arreaza, said in a tweet that the European politicians had been warned that they would not be admitted and called their arrival a "provocation."

Faiola reported from Caracas, Venezuela.