For Brazil’s embattled civil society activists, the government’s distortion of truth has taken many forms since the ultra-right militarist Jair Bolsonaro took power at the beginning of this year.

But it has hit a menacing new level after last week’s police raid on an Amazonian NGO and the arrest of four volunteer firefighters who are accused of setting alight the forest they risked their lives to protect.

Even by the standards of the past 11 months, the detentions in Alter do Chão, and the confiscation of equipment at the headquarters of Saúde e Alegria, or the Health and Happiness Project, in Santarém have proved shocking for those involved – and those who fear they may be next.

Until recently, Brazilian NGOs’ greatest fear was being publicly smeared by a president who has baselessly accused Greenpeace of creating oil slicks, environmentalists of starting forest fires and Leonardo DiCaprio of bankrolling the arson.

Now activists worry the powers of the state are being used to criminalise their activities.

“The executive has been investing in new tactics of repression,” Camila Marques from the free-speech watchdog Article 19 said during a webinar of NGOs this week. “We have never seen intelligence agencies, police and services so active in monitoring and surveillance of social movements. We are facing a scenario of arbitrary imprisonments.”

A forest fire in the town of Porto Velho, state of Rondônia, Brazil, on 9 September. Photograph: Fernando Bizerra/EPA

The case in Santarém is seen as a bellwether.

Shortly after dawn last Wednesday, half a dozen federal police armed with semi-automatic weapons showed a warrant and demanded entry to the premises of Health and Happiness Project – a long-established NGO that has worked with Prince Charles, Caritas, Coca-Cola and many other international partners.

Staff turned up for work expecting to organise medical assistance, sanitation or solar power for remote forest communities – but instead found state agents examining documents and hauling off computers and files.

“I was afraid at first, then angry,” said Jarine Reis, who is responsible for projects related to gender, tourism and traditional art. “I could taste the wrongness in my mouth.”

Members of the group felt the raid was politically motivated – and part of a broader assault on civil society by the government of Bolsonaro.

“It’s completely unacceptable in a democracy. This show of force is disproportionate. We feel violated and disrespected,” said Paulo Lima, a senior coordinator. “Unfortunately, though, at this moment in Brazil, political polarization is leading to a criminalisation of civil society.”

The mood of disbelief and fear grew stronger as news came in that one of Health and Happiness’s chief logistical officers, Gustavo Fernandes, had been arrested, along with three other men.

The four had set up their own volunteer fire brigade to help the authorities tackle forest blazes in Alter de Chão, one of Brazil’s beloved resort towns and a mecca for artists and nature lovers.

The riverside community is also a heartland of the environmental movement in Brazil.

The four men were initially denied bail and their heads were shaved. Families and friends believe they were targeted to reinforce Bolsonaro’s claim in August that environmental activists were responsible for the worst Amazonian fires in a decade – rather than his own government, which has weakened forest protections and encouraged loggers, miners and land grabbers.

Bolsonaro and the environment minister, Eduardo Salles, praised the detentions. Civil police said the suspects were under investigation for arson. Local media alleged – without proof – that they started a huge fire in September so they could take photographs for fundraising from the WWF and others.

But the fragility of the case was quickly apparent.

Federal prosecutors challenged the validity of the investigation, saying they looked into the allegations in September and found nothing to suggest any causal link between the fires and the volunteer brigade. In a public statement, they said the evidence pointed instead to land grabbers and property speculators, who use fire to clear forest and drive up its commercial value.

The case was further weakened when the suspects provided alibis that showed they were far from the fires when they started – in one case, on a plane. As doubt grew, the governor of Pará replaced the chief investigator and a judge ruled the men should be released until their court hearing.

Friends of the arrested men say all four left jobs in São Paulo so they could live closer to nature in Alter do Chão and work with tourism, permaculture and forest protection. They said the founder, Daniel Gutierrez – a 36-year-old photographer and tourist operator – worked with the authorities, received training from the state fire brigade, and went out of his way to be politically neutral.

“Everybody is in shock. Nobody can believe it,” said Priscila Cotta, who has known the men for several years. “Why would those who fight fires start fires? Everyone knows they devote themselves to the forest.”

Firefighters battle blazes in an Indigenous reserve located in Humaitá, state of Amazonas, Brazil, on 7 September. Photograph: Fernando Bizerra Jr/EPA

Others, who are reluctant to give their names due to fears of retribution, believe the raid and arrests may be linked to land speculation. Gutierrez was planning to start an organisation that would create a deforestation firewall that would prevent a sprawl of tourist resorts along the bank of the Tapajós River.

Members of Health and Happiness – which has no formal links with the volunteer brigade – have not been charged with any crime, but the police confiscation of computers and files has paralysed the group’s operations, including a hospital boat that was due to carry 40 doctors and nurses and 14 tonnes of medicine and equipment to a remote community on the Arapiuns River.

Other NGOs warn this could be just the start of a crackdown. With the government planning to tighten restrictions on public protests and relax regulations on the police, the space for civil society looks set to shrink.

The Brazilian Association of Non-Governmental Organizations said the government was pursuing a strategy to delegitimize civil society. Amnesty International’s executive director in Brazil, Jurema Werneck, said Bolsonaro’s tactics echoed similar moves in Russia, India and Hungary.

“The rhetoric of the president and other government officials creates an environment of extreme vulnerability for NGOs, activists and human rights defenders,” she said.

Those already in the midst of this assault say they have been living a nightmare.

“Each day seems 100 hours long, we never have time for anything, and we feel that someone has declared war on us,” said Caetano Scannavino, a co-founder of the Health and Happiness Project.

But he retained faith in Brazil’s institutions to right a wrong, and expressed gratitude for the many people who have given support. “We have received huge solidarity from all sides. They can take our documents, but they cannot take our joy.”