CUCUTA, Colombia (AP) — Venezuela's National Guard fired tear gas on residents clearing a barricaded border bridge to Colombia on Saturday, as the opposition began making good on its high-risk plan to deliver humanitarian aid to Venezuela despite objections from President Nicolas Maduro.

By midday, opposition leader Juan Guaido pulled himself onto a semi-truck and shook hands with its driver as he and Colombian President Ivan Duque gave a ceremonial send off to an aid convoy looking to transport nearly 200 metric tons of mostly U.S.-supplied emergency food and medical supplies from the Colombian city of Cucuta.

"Our call to the armed forces couldn't be clearer: put yourself on the right side of history," said Guaido, in an appeal to troops who constitute Maduro's last-remaining major plank of support in a country ravaged by hyperinflation and widespread shortages.

The opposition is calling on masses of Venezuelans to form a "humanitarian avalanche" to escort trucks carrying the aid across several border bridges.

But clashes started at dawn in the Venezuelan border town of Urena, when residents began removing yellow metal barricades and barbed wire blocking the Francisco de Paula Santander bridge. Venezuela's National Guard responded forcefully, firing tear gas and buckshot on the protesters, some of them masked youth throwing rocks, who demanded that the aid pass through.

Later, the youth commandeered and set afire a city bus. At least two dozen people were injured in the disturbances, according to local health officials in Urena.

The potentially volatile moment for both Venezuela's government and opposition comes exactly one month after Guaido, a 35-year-old lawmaker, declared himself interim president based on a controversial reading of the constitution before a sea of cheering supporters. While he has earned popular backing and recognition from over 50 nations, he has not sealed the support of the military, whose loyalty to Maduro is crucial.

"We're tired. There's no work, nothing," Andreina Montanez, 31, said as she sat on a curb crying from the tear gas that was used to disperse the crowd.

A single mom, she said she lost her job as a seamstress in December and had to console her 10-year-old daughter's fears that she would be left orphaned when she decided to join Saturday's protest.

"I told her I had to go out on the streets because there's no bread," she said. "But still, these soldiers are scary. It's like they're hunting us."

At the Simon Bolivar bridge, a group of aid volunteers in blue vests calmly walked up to a police line and shook officers' hands, appealing for them to join their fight.

At the same post, four National Guardsmen deserted early in the day and took refuge inside Colombia.

A video provided by Colombian authorities shows three of the men wading through a crowd with their assault rifles and pistols held above their heads in a sign of surrender. The young soldiers were then ordered to lay face down on the ground as migration officials urged angry onlookers to keep a safe distance.

"I've spent days thinking about this," said one of the soldiers, whose identity was not immediately known. He called on his comrades to join him: "There is a lot of discontent inside the forces, but also lots of fear."

Guaido, who has offered amnesty to soldiers who join the opposition's fight, applauded their bravery, saying it was a sign that support for Maduro was crumbling.

"They aren't desertors," he said on Twitter. "They've decided to put themselves on the side of the people and the constitution.....The arrival of liberty and democracy to Venezuela can't be detained."

International leaders including U.N. Secretary-General Antonio Guterres are appealing for the sides to avoid violence.

But on Friday, a member of an indigenous tribe was killed and 22 others injured in clashes with security forces who enforced Maduro's orders to keep the aid out at a crossing with Brazil.

In previous waves of unrest, citizens have been tear-gassed and killed.

Venezuelan Foreign Minister Jorge Arreaza said the military would "never have orders to fire on the civilian population" and likened the aid push to a media spectacle.

"We can only hope that sanity and good sense prevail in Cucuta, in Colombia, and that it will remain as a big show, a big party, and that they don't try to open the doors to a military intervention," he said at U.N. headquarters in New York Friday.

The push comes on the heels of a giant concert organized by British billionaire Richard Branson aimed at pressuring Maduro to accept the aid. Tens of thousands of Venezuelans gathered in a field to hear pop stars like Juanes sing beneath a scorching sun. Guaido made a surprise appearance toward the end.

"Juan arrived! Juan arrived!" people shouted as they spotted him smiling near the stage.

"Here is a Venezuela in search of freedom," he said at an aid storage facility. "Thank you, to the people of the world, for opening your doors to us."

The opposition is planning to hold three simultaneous aid pushes on Saturday. Aside from the events in Colombia, they also hope to get humanitarian assistance delivered by sea and through Venezuela's remote border with Brazil. Protests are also planned in the capital, Caracas.

Venezuela's military has served as the traditional arbiter of political disputes in the South American country and in recent weeks top leaders have pledged their unwavering loyalty to Maduro. However, many believe that lower-ranking troops who suffer from the same hardships as many other Venezuelans may be more inclined to now let the aid enter.

Opposition leaders are pushing forward in belief that whether Maduro lets the aid in or not, he will come out weakened. They also contend that if the military does allow the food and medical gear to pass, it will signify troops are now loyal to Guaido.

Analysts warn that there may be no clear victor and humanitarian groups have criticized the opposition as using the aid as a political weapon.

"I don't know that anyone can give a timeline of when the dam might break, and it's quite possible that it won't," said Eric Farnsworth of the Council of the Americas and Americas Society, a Washington-based think tank.

Fearful of what they might encounter, some Venezuelans in Cucuta said they planned to stay away from the border crossings, while others said they'd face the risks and go.

"For my son, I'd risk everything," Oscar Herrera, 25, a Venezuelan man who took an 18-hour bus ride to Colombia to buy his infant medicine for a skin irritation earlier this week.

Hernan Parcia, 32, a father of three, said he planned to go with his entire family.

"I'm pained by what's happening to my country," he said. "They can count on me."

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Henao reported from Urena, Venezuela. AP Writers Joshua Goodman and Scott Smith contributed to this report from Caracas, Venezuela.