The forum's founder Francisco Whitaker said participants would discuss ideas to create "a new civilization, based on other values."
First organized in 2001, in the southern Brazilian city of Porto Alegre, WSF developed as a counter balance to the World Economic Forum in Switzerland.
"The forum is an open university, where all ideas are legitimate. The event's proposal is in itself to stimulate plurality," said forum organizer Cândido Grzybowski.
About 120,000 people from 150 countries were set to take part in the forum's 2,600 activities.
On Thursday, they will be joined by presidents Luiz Inácio Lula da Silva of Brazil, Evo Morales of Bolivia, Rafael Correa of Ecuador, Fernando Lugo of Paraguay and Hugo Chavez of Venezuela.
At a press conference Tuesday, Italy's Rafaela Bolini – a representative from the European Social Forum – recalled that the event has for years denounced "neo-liberal globalization and the capitalist market," for which activists faced criticism and "even political repression."
"The denunciation of the dangers for humanity, for the planet and for nature was true. And now we are here because that reality needs to become visible. The solution to this crisis will not be real if it comes from the same people who created the crisis," Bolini said.
Brazilian businessman Oded Grajew, another forum founder, said the steps taken to combat the ongoing economic crisis were not aimed at a lasting solution, but were merely being enacted to save the very system that caused the current problems.
"They used to say that resources were limited. Now, in the face of the crisis, trillions of dollars suddenly showed up to save carmakers, banks and bankrupt firms – funds that would have been more than enough to combat poverty and to improve health and education," he said.
After its launch in 2001, the WSF was held in Porto Alegre again in 2002, 2003 and 2005. India hosted it in 2004, while Venezuela did so in 2006 and Kenya in 2007. In 2008 there was no one single site, but the forum was held simultaneously in 82 different countries
Mercopress