"I get threats through my website or Twitter page saying, 'Die' or, 'We're going to get you'," he says. "I just have to be careful. I can't let anyone take a photo of my face and I don't tell reporters my name."

The "Japanese Banksy" admits to being "frightened" of a government that he says is becoming more right-wing by the day.

"There have been huge changes in Japan in the past couple of years," 281 says. "The government and the police have become much more aggressive. It's more of a dictatorship."

He points to the recent passage of the state secrets law, which gives Prime Minister Shinzo Abe's stridently nationalist administration sweeping powers to keep information from the public's gaze and effectively muzzles the media. Critics have said that the full impact of the law is not clear because its very terms are, ironically, secret.

Yet Abe retains the support of a large majority of the Japanese public, riding high in opinion polls a little over a year after being elected in a landslide victory for the Liberal Democratic Party.

"I can't see why he is so popular," says 281, shaking his head. "Nobody can see what he is thinking and that is a scary thing for a leader. And I'm not surprised that people in foreign countries are wary of Japan now; Abe wants to take us back to how it was here in 1940."

If the police were to take 281 in for questioning, they could find ways to hold him - charges ranging from defamation to littering or creating a public nuisance could feasibly be levelled - but it is the fury and the aggression of the that he fears. These are the ultra-nationalists who parade through Tokyo in black vans with nationalistic slogans painted on the sides and speakers blaring out military music. On red-letter days for the extreme right, such as the anniversary of the end of the second world war, their buses disgorge files of foot soldiers in camouflage fatigues and combat boots at the Yasukuni Shrine, the final resting place of war criminals and the focus of anger from countries such as China when politicians visit to pay their respects. standard bearers hold the old wartime Rising Sun flag aloft when they turn up.

281 - if the number has any significance, he's not admitting to it - shakes his head again and changes the subject. He was not always politically motivated, he says. It was the events of March 11 three years ago that reshaped his life.

"I was at work when the earthquake struck. It was a bit like a bad dream as the building I was in swayed back and forth," he says, making snaking motions in the air with his hands. "I remember thinking that if it didn't stop soon, the whole building would be coming down."

The magnitude 9 earthquake was the most powerful in living memory in Japan and caused devastation across the northeastern Tohoku region. But much worse was to come as the massive waves that the subsea tremor had triggered rushed towards the coast.

"When the earthquake did finally stop, I watched the tsunami hitting the beaches and towns on television," he says. "It didn't seem real. I remember the TV reporters talking about the tsunami and the Fukushima nuclear plant, but I didn't really think about it very much."

Even the next day - after he spent hours walking home in step with tens of thousands of similarly stranded commuters - he was not particularly worried about the situation at Fukushima Daiichi, 281 says.

"Tokyo is a long way from Fukushima and I believed in Japan. I believed that the plant had been built safely, that they were doing everything right and that the government had things under control."

His faith slowly began to wane when he spoke with a friend working for a foreign news agency in Tokyo.

"He told me that things were very different from what we were being told on Japanese television," he says. "His company ordered their staff to stay at home and take precautions - while everyone else in Japan was going to work as if everything was just the same as always. The government told us nothing."

By April - just weeks after the second-worst accident at a nuclear plant in history - the story had been largely forgotten by domestic news outlets. And it was this apparent collusion between the government and the media to hush up the problem at Fukushima that crystallised his concerns.

"The month after it happened, people had forgotten about Fukushima, or at least no longer thought about it," he says. "Even the people who had watched on television as the reactor buildings blew up were not thinking about it. They stopped wearing face masks and being careful about the food and water they consumed."

And that frightened 281 nearly as much as the nuclear disaster itself. He began to seek out information on the internet and identified a number of Twitter posters he trusted.

"While all I could see around me was people forgetting, I found myself becoming more and more frustrated and angry that they didn't want to think about this crisis that we faced."

He wasn't completely alone in his frustration, though. An artist collective known as Chim Pom targeted the huge mural, by Taro Okamoto, which depicts the impact of the second-world-war atomic bomb on Hiroshima and adorns a wall in the capital's Shibuya station. In a corner of the 30-metre-long work, Chim Pom's artists added a panel depicting the Fukushima plant, two reactor buildings reduced to skeletal frames and belching smoke while two others are canted at crazy angles.

The incident made the news, but anyone who hoped that it might trigger a wave of projects asking similarly difficult questions by Japanese artists was disappointed.

"I was so frustrated about what was going on - about how nothing was going on - that I thought street art was the only way for me to get my feelings out," says 281. "I wanted street art to make people think and to remember again."

His first work was an adaptation of the corporate logo for the Tokyo Electric Power Co (Tepco), operator of the Fukushima nuclear plant, with the five circles in the shape of a capital T tweaked into a gas mask. He put the image up on his Twitter feed and it attracted little in the way of traffic until September, when the re-tweets went off the scale without warning.

"I felt that people overseas were more responsive to the tweets, which was good, but I felt that I was still only reaching a limited number of people, many of whom were not in Japan," 281 says. "I thought street art was the best way of getting in people's faces, of making them think."

In December, he printed out the gas mask design on a sticker at his home and travelled to the upmarket Omotesando district of Tokyo. On the pavement close to Hermès, Dior and Tag Heuer boutiques is a steel box, the contents of which route Tepco's electricity supplies throughout the district. It was quickly adorned with the first of many of 281's stylised and unmistakable images.

To date, he estimates, he has produced about 200 designs. Those that target Tepco and the government's handling of the nuclear crisis include the internationally recognised symbol for radiation overlaid with a red sprayed circle for the Japanese flag and with the simple message: "Lie nation".

The "I hate rain" series has been one of the most popular, featuring a young girl in a raincoat, presumably to protect against the nuclear fallout raining down on her. The artist won't say whether the image was inspired by a real person - perhaps his own daughter - but it is a stark and immediately understandable message to any parent.

In a rendering of the famous second-world-war image of United States Marines raising the Stars and Stripes on Iwo Jima, the flag has been redesigned to resemble the skull and crossbones symbol for poison, with the skull replaced by the Tepco logo and the legend "Occupied" written below. Another work is even more pithy; it is a black-and-white rendering of the head of Tepco on his hands and knees in apology. Underneath, in English block capitals (the language is a fashionable one for slogans in Japan) is the word "LIAR" and the company's now familiar corporate logo.

The artist's opinion on the company is fairly straightforward.

"It should be disbanded and taken over by the government," 281 says. "It has failed to take responsibility for its failings and cannot manage the situation at the plant now.

"We need to be working much more closely with other countries and companies overseas that have lots more experience and know-how than Tepco," he says. "There was Three Mile Island in the United States and Chernobyl in the Soviet Union, so there is knowledge about this sort of thing out there. But they don't want to do that."

The street artist has slapped thousands of stickers up all over Tokyo, with the designs gradually getting across a wider range of more directly political messages. There is the black-and-white image of Abe looking through a pair of binoculars, a comment on the recent secrecy law, and another a version of the American "Uncle Sam" wartime recruitment poster bearing the words "I want tax".

His favourites are three that can often be found together. Each features the prime minister with the lower part of his face obscured by a bandit-style mask; one bearing the symbol for nuclear energy in white on a red background, one in military camouflage and the third the American flag.

"Those images really provoked a reaction from the right-wingers," 281 says. "They posted messages online saying they were looking for me, that I was a criminal, and I got a lot more messages on my Twitter feed.

"I was scared," he admits. "They want me to go away and I felt there were definitely some who were intent on killing me. I believe that we need discussions of these things, to debate the situation we are in today - but they just want to silence me."

His website has been the target of a sophisticated attack by advanced systems that flooded the server, delivering tens of thousands of clicks a second to overload it and cause it to crash.

"Of course I have felt fear and I have to be careful, but I also feel empowered," he says.

He says he is coming up with designs "constantly" and his stickers have been sighted as far away as London, Paris and California. He has yet to be apprehended when putting up his works, but he takes precautions. He works at night and is careful to scout out the immediate area before leaving his mark.

What drives him on today is his pessimism about Fukushima and Japan under its current leaders.

"They're definitely not telling us the truth about what is going on at the plant," he says. "The government is easily able to hide that information and I'm sure the situation there is much worse than they have admitted.

"And now Tokyo has won the right to host the Olympic Games in 2020; there's absolutely no guarantee that this thing will have been solved by then."

One of 281's works depicts Abe in shorts and a singlet running with the Olympic torch, the flames of which are in the shape of nuclear symbols and in the colours of the Olympic rings.

Not content with devastating Japan's countryside for generations to come with radiation and with little chance of any new nuclear energy projects in the country for the foreseeable future, 281 says, the Japanese government is now staging a vigorous campaign to export nuclear technology to India, Turkey and Vietnam, among others. And then there is the matter of the government not bothering to rewrite the constitution but simply choosing to reinterpret the sections of it that get in its way, such as reversing a policy in place since the end of the war that bans Japan from selling weapons systems to other countries.

One of 281's biggest regrets is that the initial outpouring of anger aimed at the government and Tepco has fizzled out so quickly.

"Japanese people are too weak to resist the government's propaganda," he says. "They switch on the TV news and believe everything they are told. Or they don't want to think about the alternatives. I guess they just don't think there is a problem."

That attitude is very different to that of the student generation in 1960s Japan, which regularly clashed with the authorities over the issues of the day, he says. People have lost their outrage, he believes. Or it has been taken away from them by a steady stream of saccharine-sweet pop groups of prepubescent girls, an obsession with the musings of "TV talents" and reassurances from the authorities that everything is just fine.

"I don't think I'm brave at all," he says. "I'm quite the opposite; I'm shy and I never want to be in the spotlight, so that is why this sort of street art is just perfect for me.

"I love Japan," he insists, as we step out into the cold Tokyo night. "Right now, I'm really pessimistic about a lot of things that are going on here, but I really want to help this country to get better again. As an individual, I don't have much power, but I'll continue to do this as it's the only thing that I can do to help."