LATEST UPDATE: Two weeks after this story was published, the New York Times published a report indicating that the initial claims that a convoy of humanitarian aid was set on fire by forces loyal to Venezuelan President Nicolas Maduro were unsubstantiated and that video footage from the scene suggested the fire was sparked by a Molotov cocktail thrown by an anti-Maduro protester.

A U.S.-backed campaign to force President Nicolas Maduro from power met strong resistance Saturday from Venezuelan security forces who fired tear gas on protesters trying to deliver humanitarian aid from Colombia and Brazil, leaving two people dead and some 300 injured.

Throughout the turbulent day, as police and protesters squared off on two bridges connecting Venezuela to Colombia, opposition leader Juan Guaido made repeated calls for the military to join him in the fight against Maduro's "dictatorship."

Colombian authorities said more than 60 soldiers answered his call, deserting their posts in often-gripping fashion, though most were lower in rank and didn't appear to dent the higher command's continued loyalty to Maduro's socialist government.

In one dramatic high point, a group of activists led by exiled lawmakers managed to escort three flatbed trucks of aid past the halfway point into Venezuela when they were repelled by security forces. In a flash, the cargo caught fire.

Initially, some eyewitnesses claimed the National Guardsmen doused a tarp covering the boxes with gas before setting it on fire, but a New York Times analysis of video footage from the scene later disproved those claims and suggested the fire may have been sparked by a Molotov cocktail thrown by an anti-government protester.

As a black cloud rose above, the activists — protecting their faces from the fumes with vinegar-soaked cloths — unloaded the boxes by hand in a human chain stretching back to the Colombian side of the bridge.

'Humanitarian avalanche'

For weeks, U.S. President Donald Trump's administration and its regional allies have been amassing emergency food and medical supplies on three of Venezuela's borders with the aim of launching a "humanitarian avalanche." It comes exactly one month after Guaido, in a direct challenge to Maduro's rule, declared himself interim president at an outdoor rally.

Even as the 35-year-old lawmaker has won the backing of more than 50 governments around the world including Canada, he's so far been unable to cause a major rift inside the military — Maduro's last-remaining plank of support in a country ravaged by hyperinflation and widespread shortages.

Amid the standoff, Guaido was turning to diplomatic actions.

As night fell, he refrained from asking supporters to continue risking their lives and make another attempt to break the government's barricades. Instead, he said he would meet U.S. Vice-President Mike Pence on Monday in Colombia's capital at an emergency meeting of mostly conservative Latin American governments to discuss Venezuela's crisis.

'My patience has run out'

But he did make one last appeal to troops.

"How many of you national guardsmen have a sick mother? How many have kids in school without food?" he said, standing alongside a warehouse in the Colombian city of Cucuta where 600 tons of mostly U.S.-supplied boxes of food and medicine have been stockpiled. "You don't owe any obedience to a sadist ... who celebrates the denial of humanitarian aid the country needs."

A large black cloud hung over the Francisco de Paula Santander bridge as protesters passed the boxes by hand and removed them from the blazing vehicles. (Marco Bello/Reuters)

Earlier, Maduro struck a defiant tone, breaking diplomatic relations with Colombia, accusing its "fascist" government of serving as a staging ground for a U.S.-led effort to oust him from power and possibly a military invasion.

"My patience has run out," Maduro said, speaking at a massive rally of red-shirted supporters in Caracas and giving Colombian diplomats 24 hours to leave the country.

'It's like they're hunting us'

Clashes started Saturday well before Guaido straddled a semi-truck and waved to supporters in a ceremonial sendoff of the aid convoy from Cucuta. In the Venezuelan border town of Urena, residents began removing yellow metal barricades and barbed wire blocking the Santander bridge. Some were masked youth who threw rocks and later commandeered a city bus and set it afire.

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

Venezuelan President Nicolas Maduro speaks during a pro-government march in Caracas on Saturday. (Yuri Cortez/AFP/Getty Images)

The single mom 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 least 285 injured

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.

But the goodwill was fleeting and a few hours later the volunteers were driven back with tear gas, triggering a stampede.

Watch: Venezuela protests flare, soldiers defect at Colombian border

Protesters and Venezuelan soldiers cross the border into Colombia on Saturday after President Nicolas Maduro closed the border to stop the opposition bringing U.S. humanitarian aid 0:37

At least 60 members of security forces, most of them lower-ranked soldiers, deserted and took refuge inside Colombia, according to migration officials. One was a National Guard major. Colombian officials said 285 people were injured, most left with wounds caused by tear gas and metal pellets that Venezuelan security forces fired.

A video provided by Colombian authorities shows three soldiers at the Simon Bolivar bridge 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 lie face down on the ground as migration officials urged angry onlookers to keep a safe distance.

Analysts warn of no clear victor

"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. Later, he greeted five of the military members, who in turn offered a salute, calling the opposition leader Venezuela's "constitutional president" and their commander-in-chief.

A Venezuelan Bolivarian National Guard officer throws a teargas grenade toward demonstrator during clashes in Urena, Venezuela. (Fernando Llano/Associated Press)

"They aren't deserters," Guaido said. "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."

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

"Today marked a further blow to the Maduro regime, but perhaps not the final blow that Guaido, the U.S. and Colombia were hoping for," said Michael Shifter, president of the Inter-American Dialogue in Washington.

"Threats and ultimatums from Washington directed to the generals may not be the best way to get them to flip. In fact, they are likely to have the opposite effect."

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

People hurl stones at the border between Venezuela and Brazil in Pacaraima, Brazil, on Saturday. (Ricardo Moraes/Reuters)

Canadian Foreign Affairs Minister Chrystia Freeland said Saturday that Canada is "deeply concerned" with the violence in Venezuela.

"Canada calls for these unacceptable attacks to be investigated and for the perpetrators to be brought to justice," she said.

At least two people were killed and another 21 injured in the town of Santa Elena de Uairen, near the border with Brazil, according to local health officials.