The gathering of people for the swearing-in of President Barack Obama will be the largest inauguration ceremony yet staged. It will also be the biggest assembly of people for a single event in America. But more than that, it will be, if present estimates are correct, the largest political crowd in the history of the planet.

One US official told me last week that at least five million people are expected to converge on Washington on January 20, perhaps more, for the crowd is now growing in the imagination, as crowds tend to do.

That is roughly ten times the size of the population of Washington itself. It is 25 times bigger than the march on Washington led by Martin Luther King in 1963, perhaps double the biggest haj to Mecca, and at least four times bigger than the biggest previous inaugural gathering, when 1.2 million watched Lyndon Johnson take office after the assassination of JohnF. Kennedy in 1963.

The size of the expected crowd is not just a reflection of Mr Obama's popularity (and a logistical nightmare for the city authorities): it is proof of how crowds have evolved in recent history. For this will be less a political rally than an emotional pilgrimage, a collective statement about change in America that may be closest in spirit to the great crowds that assembled after the death of Diana, Princess of Wales.

The host that lined the streets for her funeral in 1997 came, spontaneously and without formal organisation, to mourn her death, but they also marked an important moment of national self-knowledge, a change in the political wind. The same will be true of the millions converging on Washington: they will come to mark the transfer of power and to applaud a new leader, but also to bear witness to change and claim a part of it, something that cannot be done by watching a television screen.

Crowds often carry grim connotations in the West, particularly in America, where the civic space between people stands in contrast to the “teeming shore” of The New Colossus, Emma Lazarus's sonnet on the Statue of Liberty. Some commentators have voiced disquiet at the people-pulling power of the President-elect: “Those vast Obama crowds,” Fouad Ajami, an expert on the Middle East, wrote in The Wall Street Journal, “recalled for me the politics of charisma that wrecked Arab and Muslim societies.” For most of the 20th century, crowds have spelt danger and demagoguery - the brutal iconography of Leni Riefenstahl's marching Nazis, the baying mob whipped up by the agitator. The 1928 film The Crowd concerned a man crushed by the conformity of the mass, the enemy of American individuality. Hitler's personal library in Berchtesgaden contained works on manipulating crowd psychology, admiringly annotated by the Führer himself.

In his classic Crowds and Power, written in 1960, Elias Canetti explored the extraordinary behaviour of the human throng - religious, military, sporting and political - simultaneously expanding and levelling. “The crowd wants to grow indefinitely,” the Nobel prize-winning novelist and philosopher wrote. “Distinctions are thrown off and all become equal. It is for the sake of this blessed moment, when no one is greater or better than any other, that people become a crowd.”

Canetti called this the instant of “eruption”, intoxicating, euphoric, and frequently terrifying. A Bulgarian-born Jew who had been forced to flee the Holocaust, Canetti wrote from personal experience of the totalitarian crowd, and focused on the violent potential of humanity in herds.

The modern crowd is subtly different. Tracing back through the evolution of non-violent protest, people gather less for ideological reasons or through “pack” instinct than for symbolic purposes, to witness an emotional event, to be present at the forging of history.

Most of those who thronged to the dismantling of the Berlin Wall had cameras, not sledgehammers. The crowd today is held together not by a Robespierre or a Mussolini but by an unnamed student in a T-shirt, holding up his hand to stop a tank in Tiananmen Square; crowds share emotions more than beliefs.

Nobody expected the vast crowds that gathered after Diana's death, just as nobody can predict accurately how many millions will flock to Washington in four weeks' time. Those crowds were motivated by grief, but also by an undercurrent of fury, aimed at the press, and at the Royal Family for failing to appreciate and share the intensity of their mood.

Looking back, those crowds marked a moment of profound change, a transition from one age to another that helped to redefine the identity of modern Britain. The multitude flexed its muscles and expressed its power insistently but without intimidation.

The ranks were filled with people who would never consider themselves part of a crowd.

The mood in Washington next month will be joyful rather than mournful, but the vast assembly will share many of those characteristics, including an undertow of anger at the ancien régime, even though many Republicans will be among the multitude.

Most of those at the Obama inauguration will see little more than the back of the person in front of them. They would see far more, more comfortably, on television. If five million do turn up, some will not get closer to the swearing-in ceremony than the Washington Beltway. None of which matters, because for many in the crowd, the main spectacle (and the central point of the occasion) will be the crowd itself.

Crowds gather to protest, to celebrate, to grieve and to destroy; but sometimes they collect, in vast and unpredictable flocks, simply to share the strange alchemy of mass assembly, to mark the blessed moment, in the words of Elias Canetti, “when all who belong to the crowd get rid of their differences and feel equal”.

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