Thousands of anti-globalisation are gathering in Paris for the opening of the second European Social Forum.
Last year hundreds of thousands marched in Florence after the forum
Around 50,000 people from 60 countries are expected to attend the event, where European Union enlargement and Iraq are expected to top the social agenda.
Four-days of conferences, seminars and concerts are expected to culminate in a large demonstration on Saturday.
Last year 300,000 people joined the march that marked the end of the first forum in the Italian city of Florence.
Security has been strengthened across the French capital, with thousands of officers at embassies, monuments and other "sensitive" sites.
French anti-globalisation campaigner Jose Bove said he hoped activists would be heard as they set out an alternate vision for the future of Europe.
"Today everything is still open and that's why debate is fundamental," he told French RTL radio.
Europe must protect us effectively from the way the United States dominates the world
"There are projects that go in the right direction, but there are big holes in social and economic areas."
The focus on EU enlargement - with next year's addition of 10 new states from eastern Europe and the Mediterranean - follows shortly after a European Commission report highlighting concerns about the countries' preparations.
Organisers say the forum is designed to allow groups such as "social movements, organisations for people without rights, unions, peace movements and citizen movements" to come together to discuss themes linked to major European issues.
Higher profile groups such as Greenpeace, Amnesty International and Oxfam are among the participants.
Other issues on the forum's agenda include anti-Semitism, distribution of wealth, the EU's policy toward Iraq and relations with Latin America.
The president of French anti-globalisation group Attac, Jacques Nikonoff, told the Le Parisien newspaper: "Europe must protect itself from American conceptions and the unipolar vision of (US President George) Bush's team."
One of the organisers, Pierre Khalfa, told French news agency AFP activists were seeking to show their opposition to "a Europe regulated by competition and the markets" under the banner of "For a Europe of rights in a world without war".
French Prime Minister Jean-Pierre Raffarin has welcomed the gathering in France "with good humour and generosity". He added that intelligent debate could spark change.
The event opens on Wednesday at 1800 GMT at four sites in downtown Paris and the
suburbs of Bobigny, Ivry-sur-Seine and Saint-Denis.