On display: the Eve Arnold images that introduced the world to a new superpower

In 1979, before China meant gleaming skyscrapers, high-speed magnetic railways and Yue Minjun canvases selling for £2m a go, the Western imagination supposed the world's most populous country to be a mass of uniform blue suits and bicycles. Opportunities for Western photo-journalists to make images that might show the reality of China after the Cultural Revolution were virtually non-existent. Political stalemate with the United States meant that Westerners found it difficult even to get a tourist visa.

Vaulting over such obstacles went Eve Arnold. Her trips to China produced some of the legendary photographer's most revealing work and a rare view of a world that few outside the country ever glimpsed.

This week her China photographs went on show as a single body of work for the first time in the UK.

By the Seventies the slightly built female Jewish-American photographer was best known for her revealing black-and-white documentary pictures of Marlene Dietrich, Joe McCarthy, Malcolm X and, in particular, Marilyn Monroe.

But Arnold, now 95, had other strings to her bow. Since 1955 she had been a member of Magnum, the prestigious co-operative of photo-journalists. She was a star photographer for Life magazine during its heyday and travelled to Afghanistan, Africa and the Soviet Union, where she grabbed shots of political dissidents in psychiatric hospitals while her official handlers were looking the other way.

To her, the distant, isolationist and all-but-undocumented China was the "ultimate assignment". For a decade she had been making annual visa applications to the Chinese embassy. Suddenly, in 1979, a rare extended visa was granted and, crucially, permission to travel around the entire country. Arnold made two long trips, covering 40,000 miles, returning with pictures that would be published in The Sunday Times magazine and her subsequent book, In China (1980). Now they are being exhibited together at Asia House in central London.

Arnold aimed at documenting "the lives of the people". The population then stood at 800 million.

In 1979 China was at the beginning of a period of industrial reform called the New Long March, an echo of the 6,000-mile trek by Mao Zedong and his troops in the 1930s which led to his takeover of power in China. The aim was to catch up in science and technology and become a world power by 2000.

"Now was the moment, now was the time to be in China," Arnold writes excitedly in the book. "They are determined not only to become a global power, but to restore to China its former significance for the world."

Arnold, who was then aged 67, hiked up to the 13,000ft Tibetan plateau, travelled across the Gobi desert and sailed the Yangtze river to capture a sunrise. She was initially "bedazzled". She visited commune after commune, and her eye began to discern the diversity of the nation.

The resulting pictures, 41 of which appear in this show, apply the humanist ethos of Magnum to the People's Republic of China as it stood on the brink of industrialisation. The agency was founded after the Second World War with the aim of protecting certain photojournalistic principles for its members, including ideals such as making positive change to the world with their images and the right of the photographer to individual expression.

Although Arnold had minders, she chose who and what she photographed, with only a few requests – the law courts and the navy – turned down.

Arnold was methodical in her scheme of work, dividing the images into categories of Landscape, People, Work and Living – the last covering birth, health matters, marriage and death. She used available light, never obstructing the all-important "decisive moment" by using flash or tripod. Arnold used colour because she felt it was more "realistic" than monochrome – a decision that might have been contentious among her peers given her membership of Magnum, which was then still synonymous with black-and-white reportage and tended to view colour work as commercial.

Her images of 105-year-old men, soldiers, athletes, doctors and students – usually at work or gesturing in a location relevant to their profession – have the ambitious aim of comprehensiveness that August Sander achieved in his pre-war portraits of Germans of every social rank.

She was not uncritical of the regime, but aimed for breadth, and In China is a social essay rather than a polemic. There are picture-postcard aerial views of Shanghai, an industrial complex on the Yangtze, snow-covered rooftops in Beijing and hundreds paddling in the brownish sand of Tsingtao beach.

"She didn't go to China to show them up," says Brigitte Lardinois, the former cultural director of Magnum and editor of a forthcoming book on Arnold's portraiture. "She wanted to know what motivated people who lived under Communism. She was open-minded and willing to engage with the people there. It was the traditional Magnum, humanist approach – lovingly depicting the world."

This is a photo essay of satisfying completeness, and is essential background viewing for anyone "bedazzled" by the bright new world of China's authoritarian capitalism circa 2007.

Eve Arnold In China is at Asia House Gallery, 63 New Cavendish Street, London W1, from 30 October to 12 January 2008