When Aleksandar Matković received the first message threatening his life, he thought it was a prank. The text, sent to his Telegram account just after midnight on 14 August read: “We will follow you until you disappear, scum.”

Matković is one of the campaigners who have been at the forefront of widespread protests against plans to develop a massive lithium mine in Serbia. He said: “At first I thought someone was joking but during the morning I got another message, saying ‘how is the struggle against Rio Tinto going?’ from another profile I didn’t know, and the app displayed the sender’s distance as just 500 [metres] away.”

Matković lives in Belgrade, but was in Split visiting a friend, which suggested to him that he was being followed. The reference to Rio Tinto, whose planned $2.4bn lithium mine in Serbia’s Jadar valley had sparked huge protests four days before, only heightened his sense of alarm.

“When I saw the distance, I was like, ‘what the hell is going on?’ The idea that someone might be following me was creepy, so I contacted my lawyer and about an hour later I received a third message. This time it was super-serious.”

The third message, written in garbled German, said: “We know about your relationship with the leaders of the revolt. We know that everything originates from you. Even if you behave notoriously and then disappear somewhere, we will follow you. We will track you and you can’t even ask the police for help because you know full well you can’t, believe me. Stay out of the public light for a while if you want to keep writing and breathing. Behave impeccably on social media. You must understand that you need to be afraid for yourself and for your little brother.”

At that point Matković reported the threats to the Belgrade police, who are now investigating, according to correspondence seen by the Guardian along with the menacing texts.

The fast-moving issues in play are complex, and have implications stretching far beyond the Belgrade public prosecutors’ office.

The campaign against lithium mines in Serbia has become a lightning rod for social grievances, mobilising tens of thousands of people for several years, including ultra-nationalists angry about economic integration with the west, environmentalists, leftists, and villagers fearful that their groundwater will be poisoned.

Serbia’s president, Aleksandar Vučić, warned last week that this opposition force was plotting a Balkan “colour revolution”.

The former Yugoslav nation has vast deposits of lithium, which are critical for the batteries used to power electric vehicles. The EU this year moved to ensure that at least 10% of the critical minerals, such as lithium, it needs are extracted in Europe by 2030.

Julia Poliscanova, vehicles and supply chains director of the thinktank Transport and Environment, said lithium was essential for powering transport electrification in Europe. “However, it is important that this lithium comes from a diverse and, above all, sustainable and responsibly sourced supply chain. Serbia can be such a partner for Europe. But the current politicisation of the planned mine is unhelpful.”

Last month a Serbian constitutional court overturned a previous ruling which had blocked the Jadar mining project. Within days the German chancellor, Olaf Scholz, visited Belgrade to sign a memorandum of understanding with the Serbian government on behalf of the EU to bring the country’s critical raw materials into Europe’s supply chain.

Matković, a green activist who co-drafted the Jadar declaration against lithium exploitation, says such agreements will tie Serbia into a “neocolonial” relationship with Europe. A few days before the death threats he received he published an open letter in the opposition Danas newspaper arguing for alternative forms of green transport.

He said: “The entire public discourse is that electric vehicles are a magic bullet which will solve climate change, but that’s not exactly true. We need more investment in public transport and infrastructure first. Then if you want to stick to electric vehicles you can use alternatives like sodium-ion batteries and hydrogen batteries.”

For its part, Rio Tinto condemned any threats of violence against Matković. A company statement said: “Rio Tinto strongly condemns any direct or implied threat of violence, online or elsewhere, against those participating in the debate about the Jadar project.”

The statement from Rio Tinto, a British-Australian multinational, said the company “also condemns in the strongest terms the intimidation and threats that our employees and their families in Serbia have faced”.

Rio Tinto employees had suffered physical threats online and when protesters picketed a community meeting, a spokesperson said.

Matković is taking no chances. Following a further slew of text threats that have left him unable to sleep he has taken steps to protect his security and that of his family, including, potentially, seeking protection in another European country’s embassy in Belgrade. He will also be requesting intervention from the UN special rapporteur on environmental defenders.

“Since 14 August my days have been a volatile mix of routine and complete chaos,” he said. “How it can be that any of this is part and parcel of our climate change mitigation strategy? What sort of a green transition is it that we want if we have to kill people to achieve it?”