It is after 3pm on a Friday when I walk into Bar do Beto, an old-fashioned Ipanema restaurant, but the place is full of diners enjoying a long, languid lunch. Glenn Greenwald is sitting in the corner, squinting crossly at his laptop. He is dressed in a T-shirt, long striped swimming shorts and flip-flops, which is something of a relief because, in the hottest Rio de Janeiro summer in decades, I am also wearing shorts.

Since the day last June when he first met Edward Snowden, a National Security Agency (NSA) analyst-turned-whistleblower Greenwald has kept up a regime of 16-hour workdays, combing through thousands of documents passed on by Snowden, writing dozens of stories for the Guardian and other papers, sniping at his critics on Twitter, and setting up a media venture with a billionaire backer. In the process, Greenwald has become, perhaps, the most famous journalist of his generation. His reputation in the US is somewhat mixed, in part as a result of his abrasive approach to political debate, but in parts of Europe and Latin America he is considered the new incarnation of Bob Woodward and Carl Bernstein.

Greenwald, 46, has lived in Rio for almost a decade, since he met his soon-to-be husband David Miranda on a holiday to the Brazilian city. “This was our place when we first met eight or nine years ago,” Greenwald says of the restaurant. “We lived a couple of blocks away and we always came here. Then they did this massive renovation and we kind of gave up on it for a few years. But we came back and the food was fantastic.”

Greenwald tells me he was in here a few nights earlier. In an event that underlines his new status as an international leftwing celebrity, the actor and playwright Wallace Shawn had invited Greenwald to see The Designated Mourner, his play about shrinking political space in a once-liberal land, when it was showing in New York last year. As Greenwald has not travelled to the US since the Snowden story broke, Shawn came to Rio to put on a performance for him. “Incredibly riveting and thought-provoking,” Greenwald says of the play, which was performed in a theatre rented for the occasion. Afterwards, Greenwald brought cast and crew to Bar do Beto to thank them.

As we open the menus, we are confronted with the paradox of Brazilian cuisine. The specialities here include sirloin steak with potatoes and rice, and feijõada, traditional stew with black beans and pork, beef and sausage. These are mouthwatering for a winter lunch but it is 37C outside and even Rio residents are complaining about the heat.

“On a day like today, you will just collapse on the street in a coma in the middle of the afternoon after a meal like that,” says Greenwald. We both order the same starter of squid rings and prawn, fried in a nutty batter, preceded by a large plate of bread with olives, cheese and tuna paste. We wash it down with cans of Guaraná, a Brazilian soft drink derived from an Amazon fruit, to which we are both addicted.

I have approached this lunch with a certain amount of trepidation. In his online persona, Greenwald has a dismissive, almost bullying approach to debate, especially when responding to journalists from mainstream organisations who, like me, have politics that he would consider far too establishment. After several papers, including the FT, reported claims last year by the US government that contradicted part of one of his stories, he slammed “#ServileDCJournalists”.

In person, however, Greenwald is charming, with a laugh that sometimes descends into a giggle, while his answers are more thoughtful than his reputation suggests. “People are sometimes not sure what to expect,” he says. “They think they are going to meet this total asshole and get screamed at.”

Some of the bluster, at least, is deliberate strategy. Greenwald is the journalist’s equivalent of an autodidact. A high-school debating champion in south Florida, Greenwald was a corporate lawyer before starting his own practice to take on constitutional and civil liberties cases – he defended several neo-Nazis in first amendment cases. Tiring of the law, he became a pioneer in using the internet to win a large audience for strong political commentary. “The way I entered the public discourse was a fairly untraditional route. I did not get my own column in the New York Times, I created my own blog on Blogspot,” he says. “In order to break through and be heard and force media figures to respond, there had to be a lot of aggression. I have a bigger platform now than I did several years ago, so I probably should have adjusted my tactics a little bit more. I don’t need to be so acrimonious and aggressive,” he continues. “But you do not always realise that your position has changed. I still think of myself as an outsider who is not at The New York Times or Washington Post, so I need to speak loudly and sometimes harshly. Part of it is personality: I enjoy the clash of ideas. I am not looking to be liked by them [Washington journalists] so the acrimony comes a little more easily for me.”

That dislike is returned in good measure by many of the media figures who have been at the sharp end of his ire and who have watched with envy as his star has risen so sharply over the past seven months.

Greenwald may still champion “outsider journalism” but he is now the frontman for one of the most lavish media ventures ever launched, First Look Media, the $250m project of eBay founder Pierre Omidyar, who has already injected a first tranche of $50m into the company. A video that Omidyar, whose net worth is estimated by Forbes at $8.5bn, released last month described the company in vague terms, an indication that it was still an evolving idea. Today Greenwald provides a little more detail. “The company will support a series of semi-autonomous digital magazines that are going to be created and shaped by a journalist or a team,” he says.

Greenwald is working with Laura Poitras, the documentary maker who introduced him to Snowden, on The Intercept, the first of these magazines. It launched this month with a story about how NSA surveillance is being used to support drone strikes against alleged terrorists. Its focus will reflect Greenwald’s interest in civil liberties and privacy, as well as reporting on the NSA: he says he is not yet halfway through the tens of thousands of documents that Snowden gave him. “It will be like a news outlet. We will be publishing daily, with new stories. I will write every day there – my column, as well as reporting stories. And there will be guest op-ed writers. It will be its own standalone media organisation.”

The other First Look digital magazines will be separate “but it will be under the same editorial structure, so we will share lawyers, we will share subeditors and technical staff”. It could, he suggests, resemble Gawker in having several standalone websites that share corporate functions.

Given how much cash Omidyar is plunging into this venture, one of the surprising things is how little he knew Greenwald. They might have had some interaction on Twitter, Greenwald says, but they had never spoken with each other before Omidyar proposed the idea. The politics of the new organisation will have a distinctly leftwing feel. As well as Greenwald and Poitras, the other marquee name hired by Omidyar is Jeremy Scahill, a ferocious critic of the Obama administration’s use of drones. A print journalist, Scahill made a documentary to accompany his 2013 book Dirty Wars, which has been nominated for an Oscar.

Beyond that, Omidyar’s intentions remain unclear. “He does not want to replicate what others are doing. He does not want to copy the New York Times or Washington Post,” Greenwald says of his new patron. “He could have bought them if that is what he wanted to do. So I am convinced that he wants to do something fundamentally different. His vision of what he wants to do entails this serious commitment to truly independent and adversarial journalism.” Greenwald adds: “What is interesting here is that Pierre is going to end up funding people he cannot control, by design.”

Even though the dishes we ordered were starters, the plates are substantial. I make my way through my squid while Greenwald, doing most of the talking, nibbles at his. Once the table is cleared, I order an espresso and Greenwald asks for Diet Coke.

He says that he left the Guardian in October last year on good terms and is very complimentary of the way the paper approached the Snowden story. “They have been pretty brave in continuing to publish,” he says. “In general, I felt that I was pretty able to do all that I wanted to do at the Guardian, although things changed a little bit once the British government started getting thuggish and intimidating, with the whole destruction of the laptops [in July 2013, Guardian staff were forced to destroy hard drives used to store its Snowden documents] and then threatening criminal investigation against British journalists who work at the Guardian . . . And of course the way they detained David at Heathrow.” (In August Miranda was detained for nine hours at Heathrow airport under the Terrorism Act. His laptop and other electronic equipment was also confiscated before he was released and allowed to fly to Brazil.)

Greenwald’s more abrasive side surfaces when the subject turns to a Guardian book, The Snowden Files, by Luke Harding, published in early February. “It is a bullshit book,” he says. “They are purporting to tell the inside story of Edward Snowden but it is written by someone who has never met or even spoken to Edward Snowden. Luke came here and talked to me for half a day without [my] realising that he was trying to get me to write his book for him. I cut the interview off when I realised what he was up to.”

Harding insists that when he spoke to Greenwald in Rio, he made it very clear he was doing research for his book on Snowden.

At the time of our lunch, Greenwald had only read extracts from Harding’s book, which he thought put far too much emphasis on some youthful anonymous online posts that Snowden once wrote. Later, after reading the whole book, he would tell me by email that it did not trash Snowden.

At the time of our meeting, though, he was clearly angry about it. “One of the things about the Guardian that I really disliked is that they used Julian Assange and WikiLeaks and got a lot of benefit from publishing the material [diplomatic cables leaked by Bradley Manning] and then completely turned into being his leading demoniser.”

Greenwald has his own Snowden book, due for publication in April, which will be his attempt to explain what to make of the NSA scandal. The scope of the data that the NSA is collecting is astonishing, as is the brazen way that it is accused of interfering with technology companies. I suggest, however, that one of the problems with the disclosures is that stories that raise major questions about privacy abuses have been mixed up with other articles that appear to do little more than reveal spy tradecraft – including the sorts of things that many Americans might expect their intelligence services to be doing.

Greenwald’s response is as combative as it is surprising. “The vast majority of the stories that people cite as having gone too far come from the New York Times or Washington Post, not from myself or the Guardian,” he says, referring to articles that looked at the way the NSA monitored Taliban fighters in Pakistan or its efforts to track communications in Iran. “Which is ironic given that we are the ones portrayed as the villains.”

It is when Greenwald expands his critique of the NSA that he starts to sound less convincing. He has made the case, in interviews and television appearances, that US surveillance is about much more than combating terrorism and is part of a broader concerted effort by the government to pursue “economic power”.

Yet the examples he cites, such as listening into a Latin American economic conference, sound less like a new front in electronic snooping and more like traditional espionage. And while one of the striking parts of the Snowden revelations is the way the US technology sector has been penetrated by the NSA, Greenwald still sounds naive about the way China works when he suggests that the links between the corporate sector and the US government are not that different to China.

A few days before our lunch, James Clapper, US director of national intelligence, had referred to some of the journalists who received documents from Snowden as his “accomplices”, while senior officials have suggested Greenwald has been selling stolen material. Greenwald has not been back to the US since he met Snowden, so I ask whether he now considers himself an “exile”.

“There is no evidence that the US government would arrest me, so it is a bit melodramatic to put that label on myself,” he says. “But every single lawyer I have talked to over the past six months and adviser with political connections has said that the chances of something like that happening, were I to return, might be less than 50 per cent but they are more than trivial. The national security state is desperate for someone to be punished for what has happened.”

We call for the bill, as Greenwald has to rush to another interview. Given Greenwald’s taste for a political scrap and flair for self-publicity, I ask if there is not a part of him that would actually relish a legal battle with the US government over what constitutes journalism? “It is easy to say in the abstract that it is a great fight to have but the cost is huge to losing that fight. So you have got to feel you are as ready as you can be,” he says. “But I am going to return, on principle, some time within the next year. I am not going to be exiled for doing journalism.”

Geoff Dyer covers US foreign policy for the FT and is a former Brazil bureau chief. He is author of ‘ Contest of the Century: the new Era of Competition with China’ (Allen Lane/Knopf)

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Bar do Beto

51 Rua Farme de Amoedo, Ipanema, Rio de Janeiro

Bread and cover charge R$26.90

Squid rings and prawn x2 R$93.80

Guaraná Zero x4 R$22.00

Diet Coke R$5.50

Coffee R$5.00

Total (including service) R$171.58 (£43.00)

This article has been amended since original publication to include a response from Luke Harding