London has been a city of immigrants since Romans began settling here in the second century, bringing with them Jews, Africans, Gauls, and other foreigners.

Over succeeding centuries, waves of immigrants, ranging from Jews and French Huguenots fleeing persecution to Irish and Indians seeking jobs, made London perhaps the world’s most cosmopolitan city – a title only New York could challenge. In earlier times, foreign workers were needed to compensate for London’s high death rate.

The rise of the British Empire, the formation of the Commonwealth that followed and the flight of people from Nazi-occupied Europe gave impetus to the flow of immigrants in the 19th and 20th centuries. More recently, there has been a noticeable rise in the number of residents coming from the 10 new Eastern European member nations of the European Union.

Today 30% of Londoners – 2.2 million people – were born outside England. More than 300 languages are spoken in the city, and at least 50 ethnic communities have populations of 10,000 or more.

This vast melange of cultures, races, and religions gives London a vibrancy, a touch of the exotic, that no other European city can match.

Some 58% of London’s 7 million people are Christians, and the city has a diverse Muslim population of more than 600,000. Two of the largest ethnic groups are black Africans and black Caribbeans. Together they number more than 700,000.

Some immigrant communities cling together, but others are scattered all over the city. London has no true ghettos, even if Southall Broadway is predominantly south Asian and Brixton is heavily black.

A British reporter, Leo Benedictus, recently spent several months charting the places in London where immigrants live, his research bolstered by the latest census figures. While many would claim that London is an unusually tolerant city, with its various races rubbing along together rather well, Benedictus concluded that indifference toward other races is probably a better description. “When hostility is the usual alternative, perhaps indifference is not such a bad thing,” he wrote in the Guardian newspaper.

Historically, immigrants have never been popular with some people. In 1185, a writer complained of London: “All sorts of men crowd there from every country under the heavens. Each brings its own vices and its own customs to the city.”

Now you hear the usual strident complaints that immigrants and asylum seekers are mostly scroungers, potential criminals, and terrorists, and this is becoming a hot political issue as an expected May election approaches. The opposition Conservative Party recently proposed quotas for asylum seekers, a daft suggestion which implies that someone legitimately fearing persecution at home could be turned away if his or her country’s quota has been filled.

The Labor government talks of cracking down on immigrants and allowing in only those who can prove they are economically useful. British businesses have long been keen to fill skills gaps with foreign workers, and Home Office-sponsored research suggests that immigrants contribute $4.6 billion more in taxes each year than they claim in welfare payments or consume by using public services.

Critics of rising immigration seem ignorant of the fact that foreign workers will be needed to help support an increasingly aged population. Today, Britain has one person over 60 for every two workers. By 2050, the ratio will be two over 60 for every three workers.

The European Union estimates that by 2030 Europe’s active work force will be 20 million short of the level needed to sustain growth and pay the pensions and health care of older citizens. But how many immigrants are too many in a crowded island nation? Can social cohesion be maintained? The debate goes on, and the political temperature is rising.

Mr. Mosely, who was recently awarded the M.B.E., was a longtime foreign correspondent for the Chicago Tribune.