CÚCUTA, Colombia - A high-stakes showdown over humanitarian aid began unfolding on Venezuela's western frontier on Saturday, with bursts of tear gas from President Nicolás Maduro's military forces and defiant vows from Juan Guaidó and the opposition to end a blockade of U.S. and other foreign aid.

Saturday's operation has been billed by the opposition and its allies in the Trump administration as a pivotal moment in its bid to topple Maduro's socialists. The attempts to haul in aid from neighboring nations are meant to test the military's loyalty by encouraging the armed forces to disobey Maduro's order to keep the aid out.

At one border crossing blocked by Maduro's forces - the Simón Bolívar bridge linking Colombia and Venezuela - the opposition plan to divide Maduro's military began to take shape with four members of the Venezuela National Guard abandoning their posts. They walked across the border as members of the Colombian armed forces wrapped fraternal arms around them.

"They have just deserted the dictatorship of Nicolás Maduro," declared Migración Colombia - Colombian's migration agency - in a statement.

A convoy of 14 trucks bearing 280 tons of aid was being prepped near a warehouse loading dock here in Cúcuta, where thousands of volunteers had camped overnight following a massive benefit concert for Venezuela put on by British billionaire Richard Branson. Guaidó - the opposition leader who claimed the nation's legitimate mantel of power exactly one month ago - was poised to lead an attempt to get the trucks over a bridge where Maduro's forces had welded containers together to physically block aid from getting across.

"Venezuela, the day has arrived in which we will take the step to enter humanitarian aid. From our borders, by land and sea, we will bring hope, food and medicines for the ones who need it the most," Guaidó tweeted Saturday. "We call everyone to go out massively to the streets in the whole country, to protest in peace at barracks, to urge the armed forces to let humanitarian aid in."

Yet after an attack by the Venezuelan military near the Brazilian border that left two civilians dead and 11 wounded, fears mounted that the attempt could be marred by further violence in this collapsing socialist state. The Venezuelan government late Friday announced the temporary closing of three key border crossings with Colombia. Just before the 8 a.m. start time for the effort to try to break the blockade, a violent confrontation broke out on the Santander bridge in the western border town in Urena - one of the crossings to Colombia ordered closed by the Maduro government on Friday.

About 200 people - a mixture of protesters seeking to bring in aid and Venezuelan workers with jobs on the Colombian side of the border - began throwing rocks at border guards, who responded with volleys of tear gas.

"They think they are the owners of Venezuela," Maria Zambrano, a 46-year-old engineer who arrived in Urena to join the aid effort, said of the guards. She said a cousin of hers with cancer cannot find medication for treatment. "But we are all united, and we will get this aid in. They won't be able to shoot us all."

In Venezuela, groups of volunteers and opposition leaders boarded early-morning buses, cars and motorbikes en route to the eastern Colombian border. In addition to vows of bringing in aid by sea and land - and via human chain if necessary - the opposition also planned large-scale rallies in cities nationwide to demand the admittance of international relief.

"I'm very concerned with the information we've received about paramilitary groups and other irregular groups already at the border with the intention to spread violence," said opposition politician Nora Bracho. "We have no doubt that there will be violence, absolutely no doubt."

In defiance of a ban against leaving the country, Guaidó made a secretive trip to Colombia on Friday to lead the aid effort and meet with regional leaders. He suggested the Venezuelan armed forces had helped him spirit across his nation's western frontier.

But he was also running the risk of being barred from reentry or arrested upon return.

The attention on Saturday remained focused on the single largest staging ground for aid in Cúcuta.

Organizers in Cúcuta had called for "every available Venezuelan" to turn up Saturday morning at 8 a.m. and await further orders.

Many began to awake before dawn on a rocky ground of a camp set up for them and began moving toward arranged meeting points to join the effort.

"We're betting everything on this," said Ricardo Justo, a 24-year-old salesman from Caracas who traveled 27 hours in bus with a group of 30. "We'll do whatever they tell us to."

Overnight, Lester Toledo, head of Guaidó's delegation in Cúcuta, addressed the crowd, telling them to have faith that the Venezuelan border guards would let them pass. Volunteers said they were prepared to make a leap of faith.

"What else can we do? I'll fight until I can't fight anymore," said Jose Antonio Pérez, 24, a health worker from nearby San Antonio. "This is the most anticipated day ever in Venezuela."

Venezuelan opposition officials say they'll be at each border crossing to meet protesters and give further instructions. The opposition leaders assured their supporters that they, and not the volunteers, would walk out front when they confronted Maduro's border guards.

Mari Rivera, 46, a Venezuelan living in Cúcuta, said she believes they have enough people to push the shipping containers over. If not, she said, they'll take the aid under the bridge and through a shallow river.

Air raid sirens on bull horns awoke the camp at 5:20 a.m., rousing them for a day of action. Folk music began playing. Kleibysad Saab, a 47-year-old Venezuelan from the state of Carabobo, lead a crowd in chants of "freedom" before offering a prayer.

"No man can close the doors that God has opened," she said. "Here we go!"

The crowd roared an amen.

Eduardo Espinel, a Venezuelan opposition politician, said the opposition would try to move aid over the border via human chains if truck convoys could not get through.

Here in the western city of San Cristobal, about 150 people divided in four buses were stationed at one of the points where the opposition called yesterday for residents to gather to be taken to the border. But around 35 soldiers with shields and a convoy were blocking the way, telling them they had orders to only let cars pass.

Three drivers decided to order people to leave their buses because they heard vehicles were being impounded en route. About 110 people began to walk up a hill toward the border, hoping they would get a ride on the way. But they were quickly blocked by armed guards, and began shouting "let us pass, let us pass." Within minutes they were allowed.

"They know which is the right side," said Julian Pozo, 54. "They are suffering too."

Many marchers wore white shirts that read "freedom for Venezuela."

"I want to free Venezuela from this yoke. The abuse has to end," said Asdrubal Castillo, a 65-year-old farmer said. "This is a historical moment. It's either now or never."

Besides the threat of military force, volunteers faced perhaps the even more dangerous possibility of violence by unruly pro-government militias, known as colectivos, as well as Colombian guerrilla groups that control large swaths of the border.

William Barrientos, an opposition politician trying to get to the border from San Cristobal, said that colectivos had already begun attacking buses overnight.

"Colectivos attacked one of the buses with our colleagues and took everything from them," he said.

He said the opposition would seek to overwhelm Maduro's forces through sheer numbers, vowing that the military would need to use force on an equally massive scale to stop them.

"And if they do, it would be a genocide," he said.

Even Maduro's foreign minister, Arreaza, sounded a note of alarm, though he said the opposition would seek to falsely blame the government for violence that it might start.

"We are worried that a situation to lament will take place tomorrow because there's many Colombian military groups at the border," he said. "Never would the armed forces shoot against the people. . . . We hope reason reigns and that this doesn't end up being a show to open the doors for a military intervention."

Ahead of Saturday's operation, opposition leaders who had arrived in the western city from Caracas were re-strategizing their trips to the border to account for higher threat levels.

After receiving information from "internal sources" of possible violence and blockades on the way, they said, they were still weighing the most secure way to proceed.

Alexis Paparoni, an opposition lawmaker, said that "if the buses are blocked, we will have motorbikes following us to continue going."

Paparoni said there were signs Saturday could be violent, including the killing of two indigenous people and acts of official repression - including the use of tear gas - against opposition politicians on their way to the border from Caracas.

But, he said, "we hope the armed forces will join our side and avoid violence."

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Baddour reported from Cúcuta, Colombia. The Washington Post’s Rachelle Krygier in San Cristobal, and Anggy Polanco in Ureña, Venezuela, contributed to this report.