But what precisely is the message, and how should governments respond to it? Much ink has been spilled in an attempt to answer those questions. However, to the unease of policymakers, they remain far from resolved.

Some observers claim the demonstrations are just the most visible symptom of a powerful grassroots backlash against globalisation, spurred by popular anxiety at the speed at which fiercer competition, capital mobility and technological advances are eroding borders and redefining the role of the state.

There is some truth in this thesis, but it is not the whole story. It does not, for instance, explain why globalisation backlash was a marginal issue in the last US presidential elections, and why candidates who sought to exploit it fared poorly.

Nor have mass demonstrations perceptibly stalled the worldwide trend towards more open, liberal and market-based economic policies. The protectionist nostrums preached by some protesters appear to have won few converts so far among policymakers.

Nonetheless, the battles on the streets do reflect deeper changes in the political environment. Many are rooted in changes precipitated by the end of the cold war.

For at least its first 30 years of existence, the Bretton Woods system was powerfully underpinned by the objective of resisting communist aggression. The prosperity and stability it set out to create were viewed in much of the world as essential to bolster capitalism and democracy.

That overarching geopolitical imperative ensured solid domestic political support - most importantly in the US - for open trade and active roles by the IMF, the World Bank and other international institutions. Because the consensus was so wide, international economic policy decisions neither required nor received much public scrutiny.

All that changed with the fall of the Berlin Wall. Robbed of their traditional rationale, multilateralist policies faced more sceptical domestic audiences in the US and elsewhere. Assent for them was no longer near-automatic, but had increasingly to be forged by building coalitions between disparate constituencies.

Simultaneously, two other developments thrust international economic decisions into greater prominence. One was the extension of trade policy beyond arcane technical matters of tariffs and quotas into sensitive areas that touched on popular nerves.

The most striking example is controversial rulings by the World Trade Organisation's strengthened disputes settlement bodies on issues including the European Union's ban on hormone-treated beef and US environmental restrictions on certain types of imports.

The second key development is the internet and other advances in mass communications. This both speeded up the dissemination of information and empowered dissident groups by enabling them to mobilise and co-ordinate worldwide. Paradoxically, globalisation's critics have harnessed one of its central driving forces in order to oppose it.

Coupled with skilful organisation and deft public relations, these tactics have enabled campaigners to raise their profile and public support. Opinion polls have repeatedly shown that in many developed countries the public trusts information from non-governmental organisations (NGOs) more than from governments or companies.

Such campaigns have not succeeded in reversing policies, they have made it more difficult to advance - notably in the area of trade. By calling into question the legitimacy of governments and international institutions, they present policymakers with a dilemma.

Some, such as the World Bank, have responded by involving environmental and other pressure groups more closely in their decisions, while the the Organisation for Economic Co-operation and Development has sought to build bridges with NGOs since its talks on a Multilateral Agreement on Investment collapsed.

NGOs have also acquired influence in the Blair government in Britain and the Clinton administration in the US. Although, the new Bush administration seems likely to be less sympathetic, it will have to contend with a Congress in which Democrats are closely linked to environmental and labour groups.

Meanwhile, Pascal Lamy, Europe's trade commissioner, insists that a new trade round can be launched only if complaints by globalisation's critics are satisfied.

But where should the lines of engagement be drawn? NGOs have played a valuable role as watchdogs, publicising dubious government practices such as tied aid and the promotion of arms sales, and campaigning successfully for measures including debt relief and a ban on landmines.

However, many of their attacks on globalisation often amount to little more than populist slogans, and the alternatives they propose are often contradictory, poorly thought out and potentially damaging, particularly to the interests of poor countries.

Furthermore, demands by some NGOs that their influence be more explicitly recognised in policy formulation raise difficult questions. Mike Moore, head of the WTO, argues that the Seattle protesters had less claim to legitimacy than, say, the democratically-elected minister from India.

Many of the demands pressed by the protesters, notably that the trade system take more account of environmental and labour standards, would be deeply divisive in the WTO, where they are strongly opposed by developing countries.

Some observers argue that efforts by governments to involve NGOs more closely are largely an insurance policy against embarrassment by awkward critics. Lord Hurd, a former UK foreign secretary, compares the phenomenon to the later Roman emperors' practice of surrounding themselves with goths and visigoths "in order to keep another lot of barbarians out".

Whether or not the tactic works, NGO activism has demonstrated one thing: that institutions and policies that are muddled or flawed now risk being exposed more swiftly and widely than ever before. The best long-term response is probably to ensure that policies are based on coherent principles, executed effectively and defended confidently before public opinion.