Ai Weiwei, Tate Modern, London

Why should millions of specks of porcelain upset the Chinese? Because Ai Weiwei never treads carefully and neither, it seems, do the patrons of Tate Modern

On the left-hand side of the far end of Tate Modern’s Turbine Hall, an invisible door, painted to blend with the wall, opens a crack and a man – a mechanic, maybe? – crunches his way across the space to a matching door on the other side and disappears.

It is both an entirely prosaic and an oddly moving moment, the latter on account of the crunching. This, the sound of shingle on a beach or perhaps of gravelled paths in a cemetery, is caused by the dislodging under the mechanic’s feet of more than a hundred million tiny artworks, almost two for every man, woman and child in Britain.

But now, from the bridge of the Turbine Hall, which is as close as you will get in future to Ai Weiwei’s great work, below is only silence. Dust raised by the feet of visitors on the first two days has led to its being closed to walkers, which is a great shame. For this latest installation in the Tate’s Unilever Series was, to my mind, the best so far.

The tiny works of art, each hand-painted in porcelain and different from all the others, are collectively called Sunflower Seeds, which is what they have been made to look like. For Ai, their maker – or, at least, the man who had them made – these have a specific significance. During the Cultural Revolution, Chairman Mao, like Louis XIV before him, had himself depicted as the sun, the faces of his 900 million sunflower people turned inexorably to his heliotropic face. At the same time, recalls Ai in an accompanying film, sunflower seeds were the one foodstuff you could be sure of finding during the frequent hungers of Mao’s reign, a token not just of survival but of sociability.

On a political level, the seeds were a symbol of repression; on a human one, they offered a rare opportunity for kindness, the sharing of a tiny plenty. In Ai's art – an art heartily disliked by the current government of the People's Republic of China, whose police have censored his work and arrested and beaten Ai himself – these two opposing sunflowers resolve into one. Sunflower Seeds is both almost pervertedly grand – 150 tons of handcrafted ceramics, covering 1,000 square metres of Tate Modern's floor to a depth of 10cm – and irreducibly simple. Each seed offers a kind of hope – of food, of kindness – and yet each is like a minute (and inedible) stone monument, so that the work is at once both lively and deathly. In those contradictions of massive numbers and individuality, of humility and grandeur, happiness and pain, Sunflower Seeds is like China itself.

Perhaps the most impressive thing about this latest Turbine Hall commission is not the number of its commas and zeros, the fact that it took the 1,600 bemused porcelain workers of the city of Jingdezhen two years to make, but Ai's own place in it. To do what he has done – to hijack the entire economy of a town, to bend the one-time makers of imperial porcelain to his conceptual will – is to behave like an emperor, or like Mao Zedong. As ever in Ai's work, the artist is centre stage, running vast risks – political, of course, but also moral, risks of image and identity. Logic tells us that these things cannot be immanent in dead clay, no matter that that clay has been moulded and decorated and baked into seeds. And yet something of Ai's own contradictions are there in this new work, in its paradoxical power.

One of these paradoxes is that there is no real need to know anything about the Cultural Revolution to "get" Sunflower Seeds, or even a need to know about China or Ai Weiwei. The man who crunched across the work when I arrived repeats the trip in reverse, right to left. Something about the potentially dehumanising scale of Ai's work humanises him: I am reminded of a peasant picking his way home through the snow of a 16th-century Netherlands landscape.

For all their maximalism, the seeds, seen collectively, are also minimalist: square-edged, in line with the taupe-and-steel brutalism of the Tate Modern's architecture. I count myself lucky to have made the short Long March across it beforepublic access was modified. As you set off, Ai's seeds shifted beneath your feet; as you looked at them, so did their meaning. They were graves and life, despair and hope. And they are still those things, and still wonderful, among the best installations I have seen.

Sunflower Seeds is no longer the social thing it was meant to be, and that, as I say, is a shame, but not fatal. In is the nature of public artworks – Richard Serra's Tilted Arc is a good example – that you cannot know how they are going to work until the public uses them. We may not see Sunflower Seeds as Ai meant us to, but that does not mean we should not see it at all. Go if you possibly can.

To 2 May 2011 (020-7887 8888)

Next Week:

Charles Darwent takes to the water with Canaletto and His Rivals