LONDON -- In 1948, London was a broken city barely crawling out of the Second World War. The Blitz had reduced whole neighborhoods to rubble. There were shortages of milk, eggs, coal and other basics. The Olympic Games, which came here that bleak summer, were about as welcome as a sunburn.

“Our country will not be able to handle the Games: it will take too long to rebuild London,” predicted one English sportswriter, as reported by author Janie Hampton in her book, “The Austerity Olympics.” “England would be jolly well satisfied never to hold the Games again.”

The Olympics went on, of course, and 64 years later, as the Games return here July 27, it is almost impossible to reconcile that gray place with today’s London: utterly rebuilt, a vibrant mash-up of tradition and innovation, a center of global finance – even though perched at the edge of a Europe in crisis – and a magnet for migrants from all over, one of the most ethnically diverse major cities in the world.

Historians describe the summer of 1948 as a transformative time for London – not just because of those Olympic Games, which built faith that the city would recover, but also because of a remarkable piece of legislation approved that July that allowed any subject of the British Commonwealth to come to the United Kingdom and enjoy full citizenship rights. The British Nationality Act paved the way for half a million ethnic minorities, primarily from the former colonies of South Asia and the Caribbean, to move to the U.K. over the next 14 years, a tide that has never receded.

“It is really a key year,” said Randall Hansen, a professor at the University of Toronto who has written about postwar London. “If you took someone from 1948 and dropped him down in London today, the most fundamental change would be how ‘black and brown’ Britain is compared to the 1940s, when it was chiefly white.”

Today 35 percent of London’s 8 million people are nonwhite, hailing from more than 90 countries, putting the city at the forefront of a demographic revolution in Europe over the past half-century. It can lay claim to being the largest Nigerian city outside Nigeria and the largest Bangladeshi city outside Bangladesh. Along its stately boulevards and redbrick lanes you’re as likely to find pubs serving fish and chips as cafes hawking chicken tikka masala, the buttery Indian curry that’s often called Britain’s favorite dish.

The city’s most iconic sporting addresses – Wembley, the northern neighborhood whose stadium hosted the 1948 opening and closing ceremonies and this year will feature soccer in a new venue, and Wimbledon, the hallowed home of tennis – boast among the most multiracial populations in London.

“People think of London and they imagine hanging flower baskets, quaint shops and people in blazers,” A.A. Gill, a columnist for The Sunday Times, said in an interview. “But if you go to Wimbledon, it is like Pune,” a metropolis in India.

Along the High Road in Wembley, shop signs are written in Tamil and Somali, a Caribbean barbershop sits a few doors down from a Bollywood-themed bar, and posters admonish pedestrians not to spit paan, the stuffed betel leaf that Indian men chew like an addiction.

“I never imagined a place like this,” said Simon Karim, an Iraqi Christian who fled Baghdad after the 2003 U.S. invasion, landed in Sweden and moved to London four years ago. His young daughter, in school here, has learned to speak with a British accent he almost can’t recognize.