Entering Ai Weiwei’s studio in Berlin is like descending into Hades. A black door on the site of an old brewery in former East Berlin opens to reveal a cramped stone staircase that plunges underground, disappearing into gloom. This sepulchral passageway deposits visitors in a small, artificially lit brick chamber.

Inside this dim room, ferocious beasts – actually, abstract sculptures fashioned by the artist from steel rods – guard the threshold, like the mythical hellhound Cerberus.

Beyond lies the nether region of the studio itself – a straggling network, spread across almost 32,000sq ft, of barrel-vaulted caverns, several filled with crated artworks.

Some of these are ready to be shipped to London, where the Royal Academy of Arts is about to mount the first survey of Ai’s career at a major institution in Britain.

Presiding over this shadowy domain is the artist himself, dressed incongruously – for someone cast in my imagination as the god of this particular underworld – in a short-sleeved blue shirt, shorts and

grey espadrilles.

‘It’s very basic,’ Ai, 58, says softly, referring to the austere spaces of the bygone brewery’s cooling cellars, which he has transformed over the past few years, despite living for a period under house arrest in Beijing.

Video Recorder (2010) Credit: Courtesy of Ai Weiwei

‘Most important, the building works as a metaphor for being underground. And I am used to that, because I have been living underground since I was a baby.’

It is less than two weeks after his arrival in Germany following the sudden return of his passport, which the Chinese authorities had confiscated and withheld for more than four years. So if anyone can be excused for indulging a bunker mentality, it is Ai – the most celebrated dissident artist (or should that be simply ‘artist’?) in the world today.

His bloody-minded skirmishes with the Communist Party of China (CPC) in recent years have won him, outside his homeland, a degree of influence that he freely admits his art and architecture would not have secured by themselves. (As well as being an internationally lauded artist, Ai has consulted for the architectural practice Fake Design, and famously collaborated on the Bird’s Nest stadium built for the 2008 Olympics in Beijing.)

"It takes a monster as big as the state to turn me, an ordinary artist, into an innocent hero"

Indeed, art is now exclusively a form of political activism for this self-appointed scourge of the Chinese government, who draws attention to the ruling party’s infringements of human rights by masterfully manipulating social media.

Everything he does as an artist, then, must be seen through the prism of politics. Even his antic music video parodying Psy’s K-pop hit Gangnam Style struck a note of resistance beneath the buffoonery: at one point, Ai brandishes a pair of handcuffs.

‘I would not separate my art from my so-called activism,’ he tells me. ‘I think I am a lively artist. And whatever I do is part of my art, and art is part of my life, so they are inseparable.’

今天，我拿到了护照。 A photo posted by Ai Weiwei (@aiww) on Jul 21, 2015 at 11:51pm PDT

This will be immediately apparent at the Royal Academy exhibition, which will concentrate on work made after Ai’s return to China in 1993, following a 12-year stint in America, spent mostly in New York. He travelled home to be with his ailing father, the poet Ai Qing, who died in 1996.

While he was in New York, Ai became versed in the conceptual art of Marcel Duchamp, who has been a defining influence: Hanging Man (1985), a metal clothes hanger bent to resemble Duchamp’s distinctive profile, will be part of the show at the RA, which made Ai an honorary academician in 2011.

‘I don’t care that much about “art”,’ Ai says. ‘Rather, I think about whether something is a good idea.’

Handcuffs (2013) Credit: Courtesy of Ai Weiwei

In China, Ai applied the tenets of conceptual art to his own subversive output. At the RA we will see marble sculptures of the surveillance cameras trained on his studio compound in Beijing by the authorities, as well as a pair of jade handcuffs, and He Xie, an installation of 3,000 porcelain crabs.

He Xie, which was also shown in Ai’s solo exhibition at Blenheim Palace last year, refers to a feast at his then newly constructed studio in Shanghai in 2010.

Organised over the internet by Ai, who was then placed under house arrest, and attended by more than a thousand guests, the dinner took place shortly before the government demolished the studio, supposedly because it had not been built with the necessary planning permission.

"It is a tradition for intellectuals to speak out against power"

That night, Ai’s guests ate river crabs, or he xie, which puns on the Chinese word for ‘harmonious’. According to Ai, in Chinese culture crabs have connotations of tyranny. The title is, of course, ironic.

For Ai, a serial provocateur, defiance is second nature. A year or two after he was born in 1957, his father was exiled for expressing liberal views, to a village on the edge of the Gobi Desert, in the remote Xinjiang province of north-west China.

Ai spent his youth there until the death of Chairman Mao in 1976, when the family was allowed to move back to Beijing and he enrolled at the city’s film academy.

S.A.C.R.E.D. (2012) based on Ai's imprisonment Credit: Courtesy of Ai Weiwei

During the Cultural Revolution (1966-76), Ai Qing was sentenced to hard labour and forced to clean public toilets. At one point, the family home was a crude earthen pit covered with brushwood.

As a result of his childhood experiences, Ai has a stubborn conviction that he must honour his family’s tradition of defiance. ‘It is a tradition for intellectuals to speak out against power,’ he says. ‘This is why artists and poets are so valued by society.’

But maintaining this tradition has come at great cost. In 2009 – a year before his Shanghai studio was razed, but shortly after the authorities had shut down his outspoken blog on the Chinese website Sina, which was reaching 100,000 people every day – Ai scuffled with police in a hotel room in the city of Chengdu after agreeing to testify on behalf of an activist friend in a court case, which would have embarrassed the government.

Surveillance Camera (2010) Credit: Courtesy of Ai Weiwei

A month later, he suffered a brain haemorrhage and was hospitalised; one of his reasons for flying to Germany this summer was to undergo a belated medical check-up on his head injury.

In 2011 Ai was detained at Beijing airport – officers placed a hood over his head – and disappeared. He was driven to a military police base in the city’s suburbs, where as well as being monitored, with the lights on 24 hours a day, by cameras fitted inside his tiny padded cell, he was accompanied at all times by two guards, who were forbidden from talking to him.

He was held, without charge, for 81 days, and later documented his imprisonment in S.A.C.R.E.D., a six-part sculpture consisting of lifelike fibreglass dioramas housed within sinister iron boxes.

A photo posted by Ai Weiwei (@aiww) on Aug 22, 2015 at 10:42am PDT

Upon his release, he was put under house arrest and prohibited from leaving Beijing for a year. Once this probation had elapsed, however, he was still not granted his passport – until, inexplicably, it was returned to him this summer. ‘I have no idea why they gave it to me now,’ he says flatly.

Within hours of receiving it, Ai had applied for a German visa so that he could see his six-year-old son, Lao, the offspring of an extramarital relationship with the documentary filmmaker Wang Fen.

For the past year, this cheerful boy, who bounces around the studio, an irrepressible foil to the hushed gravitas of his father, has been living with his mother in Berlin. ‘I wanted to protect them,’ Ai explains. ‘The Germans have always been very supportive.’

Ai's installation He Xie, which comprises 3,000 porcelain crabs, at Blenheim Palace last year Credit: Ben Murphy/Blenheim Art Foundation

If only he could say the same of the British. Anticipating flying from Berlin to London for the opening of his exhibition at the RA, he also submitted an application for a six-month business visa to Britain. The application was rejected, and Ai was offered a 20-day visa instead. ‘That was a big surprise for me,’ he says. ‘I was very frustrated.’

The reason given in a letter from the British embassy in Beijing was that in his application Ai had not declared a criminal conviction – even though, as he later pointed out on Instagram, he had never been convicted of any crime.

‘They used the same accusations as the Chinese government – accused me of being a criminal,’ says Ai, who decided to go public about the spat by posting the letter on Instagram.

A photo posted by Ai Weiwei (@aiww) on Jul 29, 2015 at 8:43pm PDT

Overnight, the Home Office backtracked, granting the artist the full visa and claiming that the Home Secretary, Theresa May, had not been consulted about his case.

It was a small victory but one that reflected Ai’s prominence. The incident also helps to explain why he finds the internet so compelling, even though Twitter is currently blocked in China.

As Ai acknowledges, the curious thing is that his stature is a direct result of his harsh treatment by the Chinese authorities – despite the fact that he has little public profile in his own country: government censorship has effectively eliminated his name from the internet in China.

"I cannot predict the fall of a party, but China will become more democratic"

In Beijing, he tells me, the officers tasked with watching his studio day and night would ask, ‘“Weiwei, are you really that famous outside China?” And I would say, “More than you can think – but you are the ones that made me become like that.” ’

He pauses. ‘Years ago, I had my show in London with sunflower seeds,’ he says, referring to his memorable installation at Tate Modern in 2010, when he carpeted the Turbine Hall with 100 million hand-crafted porcelain sunflower-seed husks, weighing 150 tons.

‘And if I walked on the street, not a single person would say,“Oh, that’s the artist.” Nobody would say that because nobody knew me.’

Sunflower Seeds at Tate Modern, 2010-11 Credit: ALAMY

Now, though, his exhibition at the RA will be international news. ‘So I would always tell the police [in Beijing], “It takes a monster as big as the state to turn me, an ordinary artist, into an innocent hero – just because I’m standing there, not scared.’’’

His words remind me of a scene from a recent BBC documentary, Big Brother Watching Me: Citizen Ai Weiwei, which tracked the artist during the fraught year following his detention in 2011.

After his detainment, Ai was ordered to pay a £1.2 million fine for alleged unpaid taxes – which his supporters believed was nothing but an underhand punishment for criticising the government so vociferously.

A photo posted by Ai Weiwei (@aiww) on Jun 27, 2015 at 5:38pm PDT

Appalled by this treatment, Ai decided to sue Beijing’s tax bureau. In the documentary, we find him on the day the case is due to be settled in 2012, sitting in front of a computer in his studio because he has been banned from court.

A colleague appears to inform him that she too has been unable to gain access to the courtroom. ‘But we are the prosecutor!’ he says with a rueful smile. ‘And it’s such a small case, isn’t it? They’ve made it into the biggest case ever. It’s strange they have to turn me into a god.’

In a sense, he is right: by gagging and goading him, the CPC has inadvertently turned Ai into a god of sorts – or, at least, a globally recognised figurehead for China’s nascent civil-rights movement.

"Young people have often told me: these are the seeds of freedom"

Meanwhile, outside the courtroom, a line of polite protesters bears witness to this fact. Each holds up for the camera a small transparent plastic bag containing one or two of Ai’s porcelain sunflower seeds. ‘We support Ai Weiwei,’ one of them says. ‘The seeds of Ai Weiwei will grow.’

Ai has reprised the powerful image in his Berlin studio. At its heart is a vast light well, opening on to a public plaza above ground. This creates a tranquil central courtyard, which is paved with ceramic tiles, each printed with a familiar design: a reproduction of Ai’s sunflower seeds.

‘These seeds will grow,’ he whispers, with zeal. ‘Young people have often told me: these are the seeds of freedom.’

Grapes (2010) made using traditional Chinese wooden stools Credit: Courtesy of Ai Weiwei

For now, he is relishing the freedom of life away from 24-hour surveillance. Is he considering a return to China in order to keep sowing seeds of dissent?

‘If I can, I will return to China,’ he says. Even after everything he has been through? ‘I have no regrets. Of course, it could be dangerous. But my father was a revolutionary and like his generation we must fight to build a better system.’

He sounds courageous. ‘I’m not courageous at all,’ he says. ‘I am naive: I have lots of ideas that are not very practical. But this is the part about myself that I most like.’

"Whatever I do is part of my art, and art is part of my life, so they are inseparable"

Surely, though, he cannot harbour optimism about China’s future in terms of human rights? ‘I do think that people, young people, still have hope,’ he says. ‘And it’s not easy to give up my belief. So if I still can, I will make an effort.

And to be there is very important.’ He pauses. ‘I cannot predict the fall of a party, but China will become more democratic. There will be a more modern society with all the important values that define a modern society.’

How can he be so sure? ‘I can never be sure. But the whole world is changing. And China, with such a large society, can only meet the challenge of the future by having essential qualities such as freedom of speech, civil rights, human rights.’

A still from Ai's Gangnam Style parody video Credit: YouTube

He speaks slowly, with calmness and authority – any trace of anger has been suppressed. ‘I used to have more rage,’ he concedes. ‘But now I am at peace. It’s like playing chess. If you understand the rules, and you see how the game is going to end, then you are not so frustrated.’

His measured delivery makes him sound like a dignitary – a powerful but subdued statesman, gathering energy for a comeback. This is, perhaps, why he was first drawn to the fortress-like architecture of his rented studio in Berlin in 2010: it provides a stronghold where he can rest before returning to the fray.

In a storage area to one side I make out hundreds of traditional Chinese wooden stools, which Ai has often used to make sculptures in his so-called Furniture series, such as Grapes (2010). Piled in rows, these three-legged stools make me think, for a moment, of an army – proxies for his ranks of anonymous supporters in China, ready to rise up.

Would he ever contemplate entering politics? He touches his beard before taking a sip of water.

"In a way I have already been a politician for a long time" Credit: Jens Umbach

‘I would not consider being a professional politician unless it was absolutely necessary,’ he says. ‘I am an artist. However, my art always relates to freedom of speech, and that is very political. So in a way I have already been a politician for a long time – fighting for essential qualities of human life.’

He rubs his eyes, apparently tired even though it is not yet 10am. He must feel depleted by the battles of recent years. Isn’t he simply too exhausted to carry on the fight?

‘If I look back, it’s very exhausting,’ he says. ‘But if you see a boxer in the ring, after three rounds, he could be beaten, falling down and losing points – but it’s not over. There are another eight or nine rounds ahead of him. And, you know, it only takes one moment – and everything can change. I still feel that way.’

Ai Weiwei opens at the Royal Academy of Arts, London W1, on September 19