Camped out on the concrete in Rio de Janeiro's swanky Leblon district, the Mídia Ninja have been watching and waiting for almost two months.

Journalists rather than assassins, they are armed with smartphones, cameras and gas masks – the tools of a fast-growing trade in street protest news.

In Leblon, they have recorded and live-streamed almost every chant, song and tussle with police in the ongoing demonstration outside the home of Sérgio Cabral, the Rio state governor and target of anti-corruption campaigners.

Elsewhere, they have been in the thick of the action in the long-running occupation of the city council, at marches to the TV Globo headquarters and on the frontlines of the protests that erupted across Brazil in June.

Though the demonstrations have shrunk and splintered, Mídia Ninja, a journalists' collective, continues to grow in popularity and influence as it provides a channel for popular discontent with politics – and the media.

Largely unheard of until a few months ago, the group claims 2,000 collaborators in 100 cities, and its Facebook page has drawn 183,000 likes.

Using social networks as a platform, it has broken news on police infiltrators and wrongful arrests – forcing the mainstream media into sheepish follow-ups.

The work is gritty, sometimes just tedious waiting and often discomfiting. There is the risk of teargas during clashes and, even in Rio, it can be cold when the winter wind blows in from the Atlantic. But the live street reporting and "no-cuts, no censorship" approach has a devoted following.

"We believe we are making a counter-narrative to show what does not appear in the mainstream media," said Rafael Vilela, a photographer, who gets no credit on his published pictures. "It's journalism based on collaboration."

Mídia Ninja has its origins in the Fora do Eixo, a group of collectives that organised music festivals and other cultural events. This largely student movement, which started in the cities of Rio Branco, Cuiabá and Londrina in 2005, has spread to more than 200 areas and encompasses an alternative university, a political party and financial system.

The movement launched Mídia Ninja – ninja is an acronym for "independent narratives, journalism and action" in Portuguese – this year as the communications arm of the movement. Its initial role was to promote gigs and run live broadcasts of concerts and conferences, but it quickly found an extra mission covering events in the favelas and small protests that nobody else reported.

When one of those demonstrations – a rally against increased bus fares – made headlines in June so did the work of the ninjas, who were among the first to collect, curate and broadcast images of police violence against the protesters. Much of the reportage was filmed on and broadcast live from mobile phones. Other material was gathered from images posted online or sent to the group.

As the protests grew to more than a million people in 52 cities during the Confederation Cup football tournament, the ninjas saw a surge in support.

Anonymous Rio, which is among the organisers of the demonstrations, look to their work. The Bar Association is collaborating with them on issues of media freedom and police brutality. In several cases, they have also led the news agenda.

Last month, Mídia Ninja sparked public indignation with images that suggested a police infiltrator might have thrown a molotov cocktail that set off a violent counter-reaction. The police deny this claim, but the coverage – later picked up by Globo TV and others – pushed the issue of provocateurs high up the news agenda, served as evidence for the defence of a wrongly arrested demonstrator and highlighted the gulf between street-level citizen journalists and big news organisations that often over-rely on police briefings for information.

Mainstream media organisations, such as Globo and newspaper Folha, have acknowledged Mídia Ninja's transformative effect.

"Folha was left in the dust," said Suzana Singer, the newspaper's ombudsman in an evaluation of the work done by the collective and citizen journalists. "It's not enough to cover protests the old-fashioned way, counting only on what your own reporters see, the police version, and images on the big broadcasters … It's necessary to take into account these new sources of information."

Even Globo, Brazil's media colossus, has started to run ninja footage and follow stories that started with ninja coverage.

"It's not our objective to make content for Globo, but it's a good sign that they use it. This really is a turning point for the Brazilian media," said Vilela. "It shows that they can't get as close as we are, though it must be hard for them to use our images, to see boys with cellphones doing better than them."

The growing influence comes with risks. Many in the group feel they have been singled out by police to try to stop them contradicting the official version of events.

"We have already taken rubber bullets, tear gas, stones, fragments of grenades. We've been sprayed with fire hoses and pepper spray and been verbally threatened. In the whole country, eight reporters have been detained and, in some cases, suffered physical aggression," said Filipe Peçanha, another ninja who was detained while filming during a protest.

A longer-term challenge is how to maintain the financial integrity of a group that wants to become a more powerful force for social change but does not want to compromise its non-commercial values.

Mídia Ninja relies heavily on volunteers, though it is trying to build a system for donations to be channelled to reporters for air fares, accommodation, equipment and living expenses.

Some supporters have offered accommodation in their homes. There is a communal wardrobe. One backer recently paid for Vilela to fly to Egypt and cover the demonstrations in Cairo. Hundreds of people have emailed the group and asked to become ninjas.

But the question of how to secure funds remains controversial inside the group. Bruno Torturra, one of the main co-ordinators, said he had given up trying to raise money via crowd-funding because of the likely backlash from members. But there are certainly no plans to cash in through advertising.

"We are making journalism as cheap as possible so that it can be more honest," said Vilela.

Additional reporting by Anna Kaiser