On Saturday, presidents, music stars and activists backing the Venezuelan opposition’s attempt to break a government blockade and bring food and medical supplies into the country, and most of the journalists covering the showdown, clustered around the border with Colombia.

But it was the remote frontier with Brazil that saw the worst violence and the boldest – though unfounded – claims of success in getting aid into Venezuela.

At least three people were killed and more than 20 injured over two days of extraordinary violence and tension in the area that saw the regional military chief briefly captured by indigenous people and the most senior local official flee into hiding.

“I am protecting the mayor and indigenous leaders,” Candy, a leader of the indigenous Pemón Territorial Guard, told the Guardian. “This flight into the forest is isn’t some kind of protest. It’s a war that they have launched, they have orders to fire at us, whoever we are.

“I don’t know if we are equipped to face them. And they want the mayor (of Gran Sabana) dead or alive, they have given the order.”

Is it constitutional to open fire on the indigenous population?

The thickly forested area is home mostly to the Pemón people, whose traditions and family ties cross a border they often treat as a mere formality. It is also increasingly dangerous on the Venezuelan side of the border, where illegal mining is expanding fast.

Tensions have been building between the Pemón, who under the Venezuelan constitution control the area as their territory, and a cash-strapped government keen to exploit rich mineral resources regardless of cost.

During the standoff over opposition plans to bring aid into the area, those strains flared into bloodshed.

The first violence came on Friday morning, when troops heading to the border killed a Pemón woman, Zoraida Rodriguez, after pro-oppostion protesters tried to stop their convoy passing.

Sixteen people were injured. Nine were taken to hospital in the Brazillian town of Boa Vista. Salomon Martinez, 40, a cartographer, travelled with them because several of his relatives were hit. Two were in serious condition, one on a breathing apparatus and a second in intensive care with gunshot wounds to his thorax and abdomen and injuries to his liver and intestine.

“We want the world to know what is happening, that the blood of Venezuelan people is being spilled and the government is trying to hide it,” Martinez said. “They are killing the indigenous on their land.”

In the wake of that attack, villagers managed to disarm and detain four soldiers: Gen Jose Miguel Montoya, military chief of Bolivar state, his driver and bodyguard and a sergeant, Grecia Roque Castillo.

“Montoya said it was unconstitutional for us to hold him, and we asked him, ‘Is it constitutional to open fire on the indigenous population?’” said state legislator Jorge Perez, who is from the community of Kumarakapay, where the incident happened.

News of the violence filtered through to the border town of Santa Elena de Uairén, which along with the nearby small military base of Roraima would become the focus of another 24 hours of bloodshed.

Demonstrators had been trying to close roads into the town since Friday night, to stop soldiers reaching the border. They failed, and the town was soon patrolled by national guard troops in armoured vehicles, who locals said shot at civilians on Saturday. The mayor of Gran Sabana, Emilio Gonzales, only the second Pemón elected to the position, fled into hiding and then over the border to Brazil.

The soldiers were watching the situation, and then the gangs started attacking from the side, injuring 10 people

Some of the worst violence was outside the Roraima base, eye-witnesses said. A group of demonstrators went to ask soldiers there to support the passage of aid, as protesters did at military bases around Venezuela. They were pushed back by tear gas from inside the base and shot at by armed men not in uniform.

“The soldiers were watching the situation, and then the gangs started attacking from the side, injuring 10 people,” said local journalist Alba Perdomo.

They were taken to the hospital in Santa Elena, which was overwhelmed by casualties from that shooting and other outbreaks of violence. Some of the injured were later taken across the border.

Legislator Larissa Acosta reported three deaths in total in and around Santa Elena de Uairén. A prominent NGO, Foro Penal, said it believed four people were killed. The opposition described the bloodshed as a massacre.

At around 6pm, locals said, another round of military reinforcements arrived in the town in armoured cars, military trucks and two buses full of people in civilian clothes carrying guns. The group freed Montoya and his team, although there was no news of the sergeant, Castillo.

Across the border, around the same time, two small trucks carrying rice, dried milk and first aid kits drove back into Brazil. Earlier in the day they crossed a few metres over the formal line of control into Venezuela, where they were welcomed by a small crowd of activists and fêted on Twitter by opposition leader Juan Guaidó.

“Attention Venezuela! The first shipment of humanitarian aid HAS NOW ENTERED through the Brazilian border,” he wrote. But it never reached anyone in need. After a few hours parked a good distance from a menacing line of Venezuelan security forces, the trucks drove away without unloading.

Brazil did play a humanitarian role – but not the one it was hoping for after a whirlwind of effort by foreign minister Ernesto Araújo, who flew to the border to join Guaidó’s “ambassador” to Brazil, Maria Belandria, for a press conference in the border town of Pacaraima on Saturday morning.

Instead of aid, its biggest contribution was providing medical treatment for Venezuelans shot by their own security forces. The situation was so volatile that Brazil’s national guard was deployed to control violence that also erupted at the border between Venezuelan demonstrators and Venezuelan national guard on Saturday and Sunday. Two Venezuelan sergeants deserted, said Brazilian colonel Georges Kanaan.

Where this leaves the Brazilian government is not clear, as it is dealing with a run of graft crises and clearly divided over its aid policy.

The far-right Brazilian president, Jair Bolsonaro, who relentlessly baited Venezuela in a bellicose election campaign, was oddly silent about his foreign minister’s efforts on Saturday, tweeting just once, in Spanish: “Strength to our Venezuelan brothers!”

“Brazil has a major humanitarian and geopolitical crisis on its doorstep but its capacity to influence the situation is very limited,” said Oliver Stuenkel, a professor of international relations at São Paulo’s Getulio Vargas Foundation, a leading business school.

“Each scenario affects Brazilian interests but in the end, Brazil is little more than a spectator.”