Venezuela's National Guard fired tear gas on residents at a bridge between Venezuela and 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 border city of Cucuta.

Guaido also said the first shipment of humanitarian aid had crossed into Venezuela from Brazil.

Get Breaking News Delivered to Your Inbox

"Our call to the armed forces couldn't be clearer: put yourself on the right side of history," he said, 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. Residents defied government orders and began removing yellow metal barricades and barbed wire blocking a bridge. Venezuela's National Guard then responded forcefully, firing tear gas on the residents who demanded the aid pass through.

Later, the youth commandeered a city bus and set it afire. 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 declared himself interim president under the constitution before thousands of cheering supporters. While he has earned popular backing and is being recognized by over 50 nations, he has not sealed the support of the military, whose loyalty is considered crucial to unseat Maduro.

Before daybreak Saturday, many national guards in riot gear forced people to move away from the road to the Simon Bolivar bridge connecting Venezuela and Colombia. The Venezuelan government had said that it was closing three of its bridges on the border.

"The armed forces have closed the borders following Maduro's orders. But the morning is just starting, and as the day goes by, more and more people will join, and I think (the government) will reflect, open the border and welcome the medicines we need," said Ronaldo Suarez, a vendor.

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 in, 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.

Since 2015, about 2.7 million people have left Venezuela as the country has grappled with an economic crisis, BBC News reports. Prices have soared due to a soaring inflation rate, leaving many struggling to afford basic needs like food and medicine.

Next week, Vice President Michael Pence will call for Maduro's ouster during a high-profile visit to Colombia, a staunch U.S. ally.

"The struggle in Venezuela is between dictatorship and democracy, and freedom has the momentum. Juan Guaido is the only legitimate leader of Venezuela, and it's time for Nicolas Maduro to go," the vice president's spokesperson Alyssa Farah said this week in a statement.