Built in the 1960s the Hayward Gallery, within the Southbank Centre complex is an uncompromising architectural feat which has housed some of the most important modern art exhibitions in London since its opening in 1968, and is now playing host to the first major exhibition by Antony Gormley to be held in the capital.

Commissioned by Hayward director Ralph Rugoff and curator Jacky Klein, in consultation with Gormely, this exhibition has unquestionably been conceived with its surroundings in mind. It feels like the sculptures have been made for the stark, unforgiving atmosphere of the gallery, and the overall impression on entering the space is that of entering a different kind of reality.

Inside the Hayward, everything in this show felt to me like it had just landed from space. The 1996 installation Allotment II fills a downstairs room with concrete units made up from the dimensions of 300 people Gormley measured in Sweden. Heads and torsos become boxed shapes, which could remind you of coffins, or cybermen, and walking between the figures, they seem at times benevolent, then suddenly malevolent. They could certainly be some kind of invading army, empty bodies with form but not substance, and they also fit perfectly in with their surroundings.

Staying with notions of sci-fi, the next room contains what could easily be the mothership which brought Allotment II down to our planet. A vast, jagged structure made of steel plates looms over visitors, threatening to fall on them any second, and then slowly reveals itself to be, bizarrely enough, a compressed foetal body. Lighting this section of the exhibition with an eerie, dim white glow is another new work, Blind Light. A room with glass walls filled with water vapour which reduces visibility to as little as two feet, this piece turns any notion of being safe indoors on its head. The water vapour means nobody inside the space can see a thing, and from outside watching people enter through the opening and just disappear is spooky enough (and reminiscent of ?Beam me up Scotty?), but when you yourself walk into it, the effect utterly disconcerting. People stumble blindly around, arms outstretched and are visibly relieved when they manage to find their way out: it?s fantastic.

The next pieces one encounters are drawings and etchings which relate to the genesis of much of Gormley?s work, and Critical Mass hangs between the space which contains Allotment II and the stairs to the rest of the exhibition. Body casts of the artist hang from the ceiling at different levels, abandoned, victims of a war from a different age, or a different world.

Upstairs there is more interactivity. Hatch is another room bringing the outside in. It is penetrated with hollow aluminium tubes so that people walking into it have to take care not to impale themselves. They come out of the walls at all different heights and lengths, and it takes a while to adjust to seeing them. Then when you leave you can look back inside the room through endoscopic tubes ? instruments for looking deep inside the body. It reminded me of a Sci Fi film I watched at Edinburgh Film Festival years ago called Cube, where the characters woke up in a room with a small hatch on each of the six walls, which led to another similar room, and so on, from which they had to escape.

The most ethereal structures of the lot are also upstairs. Matrices and Expansions takes Gormley?s bodycasting into negative space. Human forms are spun from stainless steel, which encases the barely discernible outlines of bodies, as if about to hatch them. Suspended from the ceiling these delicate structures are reminiscent of images of far-flung solar systems or stars imploding. They are have lots of presence, built up through the air and light which flows through the delicate structures; they are the antithesis to the density of the body casts which litter both the gallery and Gormely?s previous work to date.

But enough of the inside of the gallery, because it?s from outside on the three viewing platforms that one experiences the climax of the exhibition, Event Horizon, which is made up of over 30 life-size steel casts of the artist atop buildings and on walkways for 1.5sq km from the site. Some stare right down from almost on top of you, and some are barely visible in the distance, on top of offices on The Strand or the Grand Union Lodge of England. They change the entire perspective of the city, and make you look at how people stand in relation to the enormous structures and buildings we are surrounded by in the capital, and at the same time have a curious way of making the urban environment somehow less impersonal.

As the eye returns from picking out sites on the horizon, it returns to the more immediate environment, and looking up at the nearest bodies, which loom over the roof at different levels, suddenly one gets the feeling they may mean harm rather than good: Scenes from Dr Who and Invasion of the Bodysnatchers come to mind, as I realise all of these figures have their expressions trained on you. What do they want?

Turning the inside out is a common theme in Gormley?s work, but what he does in this exhibition is extend this idea to encompass both the infinite space of the universe, and the most intimate space inside our own fragile bodies. It?s confusing, and arresting and also enormously affecting. It really has to be seen to be experienced.

Blind Light is on until August 19 2007. For tickets and further information, telephone +44 (0)871 663 2501or visit www.haywardgallery.org.uk/.