Dress down so that you are not mistaken for a financial worker. Carry identification so that you are not mistaken for a violent protester. Best still, avoid the entire area of the City of London while the G20 leaders are in town.

The Metropolitan Police are preparing care- fully for demonstrations against capitalism and the financial crisis that they fear may turn ugly. Such preparations are unfortunately necessary. But more important still is that world leaders should be prepared to respond robustly to the arguments that the demonstrators are advancing.

It is easy to be distracted by the menu of colourful exhibitionism and threatening language being served up by the putative protesters. Professor Chris Knight, of the University of East London (a specialist in the revolutionary implications of menstruation rites), has told the BBC that he is planning the public hanging of effigies of bankers (while hinting that real hangings could come next). There will be the usual attempt to make an assault on the US imperialist Starbucks and McDonald's. And all will make as much sense as marching down the North Circular towards Neasden shouting “Down with the Swedish model of social democracy” before putting a brick through the window of Ikea.

Yet what is far more disappointing is the thinness of the case that the protesters are putting. It is not, after all, that protest is unreasonable. Banking regulation has been miserable, bankers' behaviour and misjudgment even more so. Given the failings of individuals and governments that preceded the current economic crisis, demonstrations and anger are an understandable reaction. But one would hope that such protests would be aimed at the right targets. And they are not.

Since the 1999 demonstration in Seattle outside a meeting of the World Trade Organisation, meetings of world leaders have attracted a loose coalition of agitators with an equally loose collection of grievances. But one theme recurs. The demonstrations attack the idea of world trade among capitalist nations. And they do so in the name of the poor and dispossessed.

To this attack, an unflinching response is required. Global free trade is not a tool of oppression. It is the route out of oppression. The rise of globalisation - a word that the critics attempt to demonise - is one of the most hopeful developments in the history of mankind.

Globalisation has helped to alleviate the poverty of hundreds of millions of people; it has allowed an unprecedented choice of goods for consumers at lower prices; it has opened up closed regimes and allowed a little light and a little liberty to shine even in places where governments have been determined to keep the blinds shut.

Global free trade allows the sharing of knowledge across the world, leading to the creation of innovative products. The protesters argue that economic growth is the enemy of the planet - that it endangers the environment. Yet from global trade comes the chance to find technological solutions to the problem of climate change. The protectionism advocated by one part of the demonstrating coalition threatens the environmental goals held dear by another part.

The German politician Joschka Fischer - himself once a left-wing street protester - has observed that if the current economic crisis had happened even just 50 years ago it might have caused much more than some carnival clowning and mindless vandalism. It might have produced war.

Why has it not? Because globlisation has made the world richer, more stable and far more aware of the extent to which we depend upon each other. A gathering of international leaders provides an opportunity to defend the very concept of international exchange against those who would seek to overturn it.