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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, with some eyewitnesses claiming the National Guardsmen doused a tarp covering the boxes with gas before setting it on fire.

Dr Andres Calle, coordinating the emergency medical response on the Francisco de Paula Santander Bridge, a short distance from the burning trucks, said that 55 people were injured by 5pm local time. The Telegraph witnessed streams of young men being stretchered in; one had lost an eye, another was shot in the chest with lead pellets.

Colombia said a total of 285 were wounded at Venezuela's border crossings.

The most serious incident came hundreds of miles away, at the Santa Elena de Uairen crossing point on the southern border with Brazil.

Two people were killed and 31 wounded when troops blocking the entry of aid opened fire on civilians hoping to gather it, according to rights group Foro Penal.

"They are massacring the people of Venezuela in Santa Elena de Uairen and San Antonio, where from seven o'clock in the morning they did not allow Venezuelans to gather to bring in humanitarian aid," Guaido told reporters in Cucuta where he was coordinating the aid operation.

"From that moment, they deployed irregulars on Venezuelan territory, firing weapons to try to stop what is inevitable to stop," he said.

View of vehicles burnt during protests on the eve in Santa Elena de Uairen, Venezuela -in the border with Brazil credit: AFP

Gaby Arellano, the opposition deputy chosen by Mr Guaidó to lead the convoy across the Francisco de Paula Santander bridge – one of four bridges linking the Colombian town of Cúcuta with Venezuelan communities on the other side – told The Telegraph that they tried to get five lorries across.

Two were burnt to a cinder and two were stolen by Mr Maduro’s forces. One returned, she said – and as she spoke shots rang out across the valley, as volleys of tear gas were fired, sending the crowds running. Gunfire could also be heard in the distance.

Similar scenes played out on another of Cúcuta’s main crossing into Venezuela – the Simon Bolivar bridge. Venezuelan forces fired tear gas into the crowd, sending the thousands of volunteers running.

Mr Guaido said on Saturday night that Mr Maduro had violated the Geneva Convention by blocking the aid.

Self-declared acting president Juan Guaido has vowed humanitarian aid would enter Venezuela despite a blockade credit: AFP

Elsewhere in Venezuela, the country was in turmoil. Military officers defected and thousands were in the streets of Caracas both for and against Mr Maduro, who danced on stage at a “Hands off Venezuela” rally.

Barracks across the country were surrounded by Mr Guaidó’s supporters, entreating the soldiers to defect.

“Put yourself on the right side of history,” said Mr Guaido.

Angered by Colombia's support for Guaido, Mr Maduro announced Venezuela was severing diplomatic ties with Bogota, and gave Colombian diplomats 24 hours to leave the country.

Gonzalez Pons, a Spanish MEP who was expelled from Venezuela last week, told The Telegraph he was there to be “the eyes of Europe”. He added: “Maduro needs to know the world is watching.”

Mr Guaidó’s supporters were overjoyed by his declaration that an aid convoy had got through in Brazil, although reporters on the border later countered the claim, saying the aid was stuck in no-man’s land.

At least one member of the feared FAES squadron and seven members of the national guard, based on the Colombian border, switched sides to pledge allegiance to Mr Guaidó – three of them, on the Simon Bolivar bridge, driving their armoured vehicles across to shove blockades out of the way.

How we got here

Mr Maduro had ordered the Venezuelan military to close the border with Colombia to bar humanitarian aid, which is being supported by the opposition, from entering the country.

As many as 300,000 Venezuelans are in dire need of food and medicine after years of shortages and malnutrition, according to Guaido. The country is gripped by a humanitarian crisis that has seen poverty soar during a prolonged recession.

United Nations figures show that some 2.7 million people have fled Venezuela since 2015 amid the crisis, and some 5,000 Venezuelans emigrate from their country each day.

Mr Guaido, recognised by most Western nations as the country's legitimate head of state, invoked articles of the constitution in January to assume interim presidency and denounced Mr Maduro as a usurper, arguing his 2018 re-election was illegitimate.

The opposition leader was attempting to cross into Venezuela from the Colombian border with thousands of volunteers carrying emergency supplies.

While the need for basic food and medicines is real, the effort is also meant to embarrass military officers who continue to support Mr Maduro's increasingly isolated government.

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What happens next?

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 Mr Maduro.

However, Mr Guaido’s supporters have been buoyed by news of aid convoys getting through in Brazil, and by multiple defections by Mr Maduro’s forces.

Mr Maduro, who has support from China and Russia, accuses the United States of plotting a military intervention and using aid as a "convenient pretext for conducting military action."

Mr Maduro also broke off diplomatic relations with Colombia on Saturday and said he would expel the ambassador.

04:45AM Pompeo calls on security forces to do 'right thing'

U.S. Secretary of State Mike Pompeo is calling on Venezuelan security forces to "do the right thing" by allowing humanitarian assistance into the country.

In a statement Saturday night, Pompeo also said the security forces could protect civilians against what he called the "armed gangs" of President Nicolas Maduro and support the Venezuelan constitution and the rule of law.

Pompeo said, "Now is the time to act in support of democracy, and respond to the needs of the desperate Venezuelan people." He also repeated a warning from the U.S. government that those who oppose the peaceful restoration of democracy will be held accountable.

A burnt truck previously loaded with humanitarian aid sits on the Francisco De Paula Santander International Bridge near the border with Venezuela in Cucuta credit: Bloomberg

04:09AM Rubio: Violence opens door to new options

Marco Rubio, the Florida Senator, has echoed the remarks of Juan Guaido, in an apparent suggestion that military intervention remains an option.

03:22AM 'All options open'

Venezuelan opposition leader Juan Guaido says he will ask the international community to keep "all options open" in the fight to oust Mr Maduro from power.

Guaido's call came after a turbulent day in which a US-backed campaign to send humanitarian aid into Venezuela met strong resistance from security forces who fired tear gas on protesters, leaving two people dead and some 300 injured.

Late on Saturday, the opposition leader tweeted; "Today's events have obliged me to take a decision: To propose in a formal manner to the international community that we keep all options open to liberate this country which struggles and will keep on struggling."

Guaido, who declared himself interim president, is recognised as so by the US and some 50 nations.

02:07AM Guaido to meet Pence

Juan Guaido, the Venezuelan opposition leader, says he will meet US Vice President Mike Pence at a meeting on Monday of regional diplomats.

The emergency meeting of foreign ministers from the so-called Lima Group of mostly conservative Latin American nations was organised to discuss Venezuela's crisis. It will take place in Colombia's capital of Bogota.

Mr Guaido, whom the US and some 50 nations recognise as Venezuela's rightful leader, spoke from Colombian city of Cucuta alongside President Ivan Duque.

Venezuelan opposition leader Juan Guaido, talks to the media during a news conference, in Cucuta credit: Reuters

While insisting he wouldn't give up in his fight to deliver the aid, he didn't ask supporters to continue risking their lives and make another attempt to break the barricades set up by Mr Maduro's socialist government.

But he did make one more appeal to troops to join the opposition's fight for power.

"How many of you national guardsmen have a sick mother? How many have kids in school without food," he said, standing alongside a warehouse where some 200 tons of mostly US-supplied boxes of food and medicine has been stockpiled.

01:14AM Pompeo condemns violence from Maduro's 'thugs'

US Secretary of State Mike Pompeo condemned an outbreak of violence he said was perpetrated by Venezuelan President Nicolas Maduro's "thugs" after security forces fired on demonstrators, killing two people and wounding more than 300.

"The U.S. condemns the attacks on civilians in #Venezuela perpetrated by Maduro's thugs. These attacks have resulted in deaths and injuries," Pompeo wrote on Twitter.

"Our deepest sympathies to the families of those who have died due to these criminal acts. We join their demand for justice. #EstamosUnidosVE."

12:40AM 'We're living a human catastrophe'

Tensions are running high in the city of Santa Elena on the Brazil-Venezuela border.

Thousands remained at the Venezuelan city's international border crossing with Brazil to demand the entry of food and medicine as dusk fell, AP reports.

Two trucks carrying humanitarian aid are stuck at the crossing, which has been blocked by the Venezuelan National Guard.

People look on as tear gas this thrown in Santa Elena De Uairen

Many Venezuelans on Saturday sang their country's national anthem and demanded that Mr Maduro let the aid through.

Pastor Djalma Justino Alves, 52, said he had never seen such a desperate situation.

"It's very tense, we're living a human catastrophe," he said.

12:07AM Aid boat threatened by Venezuelan military

A boat carrying aid from the US territory of Puerto Rico had to turn back after receiving a "direct threat of fire" from Venezuela's military, Governor Ricardo Rossello said.

He called the move a "serious violation against a humanitarian mission" that is "unacceptable and outrageous."

11:13PM 285 wounded in aid standoff

Colombia says 285 people have wounded at border crossings with Venezuela.

11:12PM 60 Venezuelans defect

60 Venezuelan security forces have defected to Colombia, AP report

11:10PM Trucks return to Colombian warehouse after failed aid effort

At least two trucks carrying food and medicine for Venezuela have returned to warehouses in Colombia after being turned back from the border by troops loyal to President Maduro fired teargas.

At least two trucks caught fire on the Simon Bolivar bridge linking the two countries after a convoy carrying aid was met by Venezuelan troops who repelled the attempt to cross.

As well as the two trucks that returned to warehouses in Cucuta, a further two trucks left the area of the bridge but their destination was not immediately clear, a Reuters witness said.

09:59PM Maduro: "Yankee go home'

Mr Maduro denies his oil-rich nation has any need of aid and accuses opposition leader Juan Guaido of being a coup-mongering puppet for Donald Trump.

Supporters of President Maduro take part in a march in Caracas credit: AFP

"What do the Venezuelan people think of Donald Trump's threats? Get your hands off Venezuela Donald Trump. Yankee go home," Mr Maduro told a rally of red-shirted, flag-waving supporters in the capital, Caracas.

Along the Colombian border, Venezuelan security forces halted the convoys with a barrage of teargas. At the crossing by Urena, two trucks caught fire, sending plumes of dark smoke into the air, while crowds started removing boxes of supplies, a witness said.

09:50PM At least two killed in aid standoff

At least two people were killed and trucks loaded with foreign aid were set ablaze after Venezuelan President Nicolas Maduro deployed troops and armored vehicles to turn back humanitarian assistance at border crossings with Colombia and Brazil.

At least two people were killed, including a 14-year-old boy, and 31 wounded during clashes at the border between Venezuela and Brazil, where the Venezuelan military blocked the entry of humanitarian aid, a human rights group said.

"The two deaths are the product of the military's repression during clashes in [the southern town of ]Santa Elena de Uairen. Both of them died from bullet wounds, one of them in the head," said Olnar Ortiz, an activist with the Foro Penal rights group critical of President Nicolas Maduro's government.

On Friday, a married couple in a nearby indigenous community were shot dead by security forces.

08:23PM Aid trucks set alight

Venezuelans are rushing to rescue boxes of emergency food and medicine from burning trucks stalled on a bridge to Colombia.

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.

Opposition supporters unload humanitarian aid from a truck that was sent on fire after clashes with Venezuela's security forces at Francisco de Paula Santander bridge

Fernando Flores, an eyewitness who described himself as a lawmaker from Ecuador, said national guardsmen acting under orders from Nicolas Maduro had torched the trucks once they crossed into Venezuelan territory.

Mr Maduro has vowed to block any aid shipments, considering them a "Trojan horse" intended to pave the way for foreign military intervention.

07:25PM Maduro: Venezuela breaking diplomatic relations with Colombia over aid

President Nicolas Maduro said on Saturday his government had broken relations with Colombia and would expel some Colombian diplomatic staff after Colombia assisted the opposition's efforts to bring humanitarian aid into the country.

"Patience is exhausted, I can't bare it anymore, we can't keep putting up with Colombian territory being used for attacks against Venezuela. For that reason, I have decided to break all political and diplomatic relations with Colombia's fascist government," Mr Maduro said in a speech.

He said the ambassador and consular staff would have to leave Venezuela within 24 hours.

07:10PM Another significant defection

The first member of the feared FAES shock troops announced that he too was defecting, Harriet Alexander reports. William Cancico said he was "tired of seeing my people suffer." He added: "I won't turn against them."

Colombian police escorting a Venezuelan soldier who surrendered at the Simon Bolivar international bridge credit: AP

05:20PM Aid for Venezuela to be passed by human chain from Colombia

A convoy of trucks carrying humanitarian assistance for Venezuela will be unloaded at the Simon Bolivar bridge on Colombia's side of the border and the aid will be transported by a human chain across the frontier, Colombia's migration agency said.

05:14PM Venezuelan troops unleash tear gas on protesters

Venezuela's National Guard have fired tear gas on residents clearing a barricaded border bridge to Colombia.

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.

Venezuelans hold a protest in the border city of Urena credit: AFP

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.

"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."

Venezuela's opposition lawmaker Jose Manuel Olivares and supporters march toward the Simon Bolivars bridge on the outskirts of Cucuta credit: Reuters

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.

Four National Guardsmen deserted the force 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. He called on his comrades to join him: "There is a lot of discontent inside the forces, but also lots of fear."

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 UN headquarters in New York Friday.

04:54PM Venezuelan army major recognises Juan Guaido as leader

In another blow for Mr Maduro, a major in the Venezuelan army, Hugo Enrique Parra Martínez, has publicly recognised Juan Guaidó as Commander-in-Chief of the National Armed Forces, according to reports.

04:48PM 'Usurper Maduro will be responsible for any violence'​

Ivan Duque, the Colombian president, has called for warned against Venezuelan troops blocking the supplies leaving from his country, saying it would be a breach of human rights laws.

“We demand that it be permitted to pass in a peaceful manner into Venezuelan territory for the benefit of those who need it,” he said in a video posted by a journalist.

If the aid is denied entry, he added, “the usurper Nicolas Maduro will be responsible for any act of violence.”

04:26PM How long will Venezuelan troops stay with Maduro?

The deployment of aid by the US and the Venezuelan opposition on the border with Colombia is a high stakes game to test the loyalty of Venezuela's armed forces - effectively asking them to chose between alleviating suffering or staying faithful to the regime.

President Nicolas Maduro has ordered the troops to bar the entry of the aid convoys. But the majority of Venezuelan security forces ready to turn on President Maduro, Coddy Weddle reported from Caracas recently. Read the full story here.

04:22PM Protesters plead with soldiers

Meanwhile in the Venezuelan capital, protesters are heading to a military airport to beg soldiers to defy President Maduro and allow aid into the economically-stricken country.

04:18PM More aid trucks head to Venezuela

The Telegraph's Harriet Alexander is following the progress of aid trucks leaving from Cucuta, Colombia which are attempting to cross into Venezuela.

Juan Guaido, self-declared interim president, appears to be riding the truck to the border.

04:11PM First aid shipment arrives in Venezuela

A truck carrying humanitarian aid crossed into Venezuela from Brazil at midday on Saturday, opposition lawmaker Miguel Pizarro told reporters in Caracas.

"This is a great accomplishment, Venezuela!", opposition leader Juan Guaido says in a tweet.

A Reuters witness said, however, that while the truck was on Venezuelan soil, it had not yet passed through the customs checkpoint.

04:02PM Tensions flare in Urena, Venezuela

A bus that was torched during clashes with the Bolivarian National Guard burns in Urena, Venezuela credit: AP

Clashes have intensified in Urena, the Venezuelan side of the border with Colombia. Venezuela’s national guard fired tear gas on residents clearing a barricaded border bridge between Venezuela and Colombia.

Protesters responded by stealing a red city bus and setting it on fire, according to reports on the ground. The bus has now set light to a nearby building. Flames from the bus also caused nearby power lines to spark.

Demonstrators destroying a city bus in Urena credit: AP

03:31PM Brazil's humanitarian aid has arrived on the border

The first truck with humanitarian aid from the Brazilian government has arrived in the city of Pacaraima on the border with Venezuela.

The crossing has been closed on orders from Venezuelan President Nicolas Maduro and the truck loaded with food and medicine will now wait in Brazilian territory.

Brazil's Foreign Minister Ernesto Araujo said he expects Mr Maduro's government to allow the aid to pass.

"It is very exciting to see people anxious to recover their freedom and have a decent life," Mr Araujo said.

03:06PM Venezuelan opposition leader arrives at the border

The Venezuelan opposition leader has arrived at the border with the leaders of Colombia, Chile and Paraguay.

03:03PM Defected soldiers named

We now have the names of three of the four national guard members who defected earlier this morning, Harriet Alexander reports from the border.

Three drove two tanks across the Simon Bolivar bridge, shunting the barricades out of the way to hand themselves in. The fourth walked across the Francisco de Paula Santander bridge, to huge applause.

The three have been named as Lt Richard Sánchez Zambrano, Sgt Major Edgar Torres Valera and Sgt Major Oscar Suárez Torres.