By the BBC's Tom Gibb in Porto Alegre
While the world's economic leaders meet in New York, their opponents kicked off their own rival conference in the southern Brazilian city of Porto Alegre with a display of music, dance and slogans against US military actions and capitalist globalisation.
The World Social Forum - first held in the city last year - grew out of the mass protests and street battles with police in cities such as Seattle and Prague.
But it is now having to redefine its aims and tactics in the wake of the 11 September attacks on America.
Either we will have a world without wars, or we will not have a world
Noam Chomsky, MIT professor and delegate
Last year the main theme was how to stop the spread of America's wide free trade agreement.
Many argued that free trade as defined by those with power undermines local democratic institutions in favour of multi-national corporations.
While this aspect is still very much on the agenda, many this year are more concerned about stopping war.
Chomsky: US 'ruthlessly extending' global control
"Either we will have a world without wars, or we will not have a world," writer and key note speaker Noam Chomsky told delegates at the conference.
Many at the conference are also saying that the 11 September have made things more difficult.
"Through history there has never been a terrorist act which has helped the left," complained Luis Ignacio da Silva, or Lula, the leader and presidential candidate for Brazil's Workers Party, which is hosting the forum.
He condemned both the attacks and the US response, saying that the Bush administration should have used political methods to punish those responsible rather than going to war.
Mr Chomsky went even further, accusing the United States and its allies of using 11 September as a way of undermining opponents and ruthlessly extending control round the globe.
"The message is that we the powerful will pursue our own agendas even more relentlessly than before, while you the rest of the world are supposed to be quiet and submissive and obedient and not raise your voices," he said, while urging people to not pay heed to such sentiments.
There is similar concern about growing US intervention in Latin America.
Globalisation has prompted mass, often violent, protest
The US and the IMF are now laying down the rules for bankrupt Argentina to get vital aid during the worst economic and social crisis of the country's history.
However, there is alarm that the United States has branded the FARC guerrillas in Colombia as a terrorist organisation and appears to be considering more direct intervention in the decades-old civil war.
"My hope last year was that the people of the United States (would) stop the plans of war," said Colombian peasant leader Hector Mondragon.
"But after 11 September, I think this is impossible.
"The activists for peace are a minority in the United States where there is a nationalistic war hysteria."
He himself lives in hiding much of the time after repeated death threats from the police and paramilitary groups.
He said that many of his colleagues have been murdered.
While the US war on terrorism has heightened fears at Porto Alegre, it has also made it harder to hold mass protests against the same institutions attacked by Osama Bin Laden, especially if they lead to violent confrontations with police.
Adelaide Giuliani (L) whose son died during protests in Genoa last year
This year organisers of the forum are making a clear effort to distance themselves from all types of violence.
Guerrilla groups such as Colombia's FARC or the Basque separatists ETA have not been invited.
Anarchists who have led street battles at a number of recent world summits are also not taking part.
"This is a peaceful movement and we cannot accept anything other than that," said organiser Maria Luisa Mendonca.
The conference is being attended by hundreds of non government organisations: aid groups, church leaders, environmentalists, women's organisations, trades unions, political parties and others.
With such a disparity of voices, there will be no attempt to produce a final unifying document.
Rather on each of the four days the main debates will be summarised.
However many are keen to this year come up with concrete proposals which can form the centre of future campaigns, and they will no doubt judge the forum's success on these grounds.