Velázquez with a Leica

Even if the name is unfamiliar, you'll probably know some of the iconic images created over the past decade by the Brazilian photojournalist Sebastião Salgado. In the gold-mines of his native land, labourers swarm over blasted earth like a scene out of a Bosch inferno. On a dam project in the deserts of Rajasthan, women workers wield picks like sari-clad furies. In the teeming camps of central Africa or the Balkans, refugees from the Rwandan genocide or the Serbian expulsions slump and huddle in endless modern Pietàs or Depositions, dramatically lit and framed against turbid skies.

Salgado travels into the deepest circles of hell that the politics and economics of a disordered world can dig, and returns with images of a surpassing, even theatrical, beauty. Since the late Eighties, he has emerged as the leading successor to the heroic documentary photographers of the mid-20th century.

Born in the rural Brazilian state of Minas Gerais, and trained as an agronomist, he fled to Europe with his wife from a military regime in 1969, "part-refugees, part-immigrants, part-students". He had never even dabbled in photography before 1971, when the International Coffee Organisation sent him to Africa to assess a plantation, and he brought his wife's camera along. So his first snaps were taken (with a resounding irony) in Rwanda.

Today, Lélia Wanick Salgado directs Amazonas Images in Paris, the support operation that funds and co-ordinates her husband's global documentary projects and shorter trips to crisis zones on assignments from newspapers and magazines. In 1993, he completed Workers, a vast "archaeology of the industrial age" that depicted the new faces and sites of manual labour, from the ship-breakers in Bangladesh to the Channel Tunnel teams. This month sees the publication by Aperture of its successor: Migrations, the monumental outcome of visits to 40 countries over six years in search of the diversity - and unity - of "humanity in transition".

As he moved from the slums of Manila to the camps of Croatia, from Russian Jews on the promenades of Brooklyn to country folk adrift in booming Shanghai, Salgado ran across so many children that their faces fill a separate book, The Children: refugees and migrants. "Pure energy surges from them even in the worst of circumstances," he says. Yet, whenever they agreed to pose for him, a brooding stillness would descend. These young Kurds, Angolans, Bosnians or Sudanese fix the spectator with an almost aristocratic sense of grace and guarded dignity. It's as if Velázquez had turned up on the front-line with combat fatigues and a Leica.

As a book, Migrations has the same hefty tombstone grandeur as Workers, though its contents will also be seen around the world in a travelling exhibition. Once more, Salgado refuses to caption the pictures as they appear. Instead, he provides a meticulously detailed booklet that explains the background to every image.

The first section of Migrations flits from country to country, deliberately blurring (as does the entire project) that theological distinction between "asylum-seeker" and "economic migrant" that looms so large in current British politics.

The second, and grimmest, chapter focuses on the traumas of central Africa. After a tour of the pullulating metropolitan favelas and peasant land-invasions of Salgado's home continent, the voyage ends in the sprawling (but somehow hopeful) megacities of the East - Istanbul, Shanghai, Bombay, Manila, Jakarta. The whole thing begins, biblically, with small boats crossing a river: the Suchiate, which divides Guatemala from Mexico and forms the first obstacle on the route to "el Norte". It ends with dawn on the Bund in Shanghai, as proud new skyscrapers and calm morning exercisers hint at the fulfilment of a dream of plenty cherished by every questing figure in the book.

Throughout the book, Salgado revels in his unique talent for the orchestration of crowds, landscape and weather-effects into stirring visual symphonies. This flamboyant aestheticism makes his a controversial, as well as a celebrated, eye. Detractors accuse him of knitting sentimentalism and self-indulgent gestures into a style of "grandiose overstatement".

Yes, these pictures ransack the iconography of the Western fine-art tradition with a shameless disregard for all the post-modern virtues - indirection, irony, self-analysis. Yet the frank pursuit of pictorial beauty also plays lavish tribute to the wretched of the earth - the people who fill every frame. On the dusty road, around the bleak camp, in the fetid shanty town, we glimpse a sort of Utopian splendour. This is an art of dreams as well as documents.

Sebastião Salgado's 'Migrations: humanity in transition' and 'The Children: refugees and migrants' are published by Aperture (£65 and £30 respectively)