The South American trade pact Mercosur has ended its summit with 10 presidents agreeing to work together to boost trade, create jobs and cut poverty.
Mr Chavez says Mercosur has much to learn from Cuba
Five of the heads represented Mercosur members while others such as Bolivia's Evo Morales and Cuba's Fidel Castro attended to show their support.
Venezuela's Hugo Chavez hailed his country's formal entry into Mercosur with a call to tackle social ills.
Thousands of activists descended on the summit venue, Cordoba in Argentina.
They gathered after the summit in the grounds of Cordoba's university to hear Mr Chavez speak.
Capitalism is perverse, he told the summit - if you are rich, you get looked after but if you are poor, you die.
The BBC's correspondent, Daniel Schweimler, says the promises announced at the summit - to increase trade and employment and bolster democracy - are the kind always made at such meetings.
But, he says, this year was different in that the presidents in their suits and the banner-wielding demonstrators on the streets wanted much the same thing.
Venezuela formally joined the 15-year old customs union at the summit.
Mr Chavez said that Mercosur, which has been undermined by internal disputes for much of its history, was entering a "new stage".
He urged it to stand firm against US free-market policies in South America, which he argues have weakened the continent's economic clout.
He called for greater emphasis to be given to reducing poverty and creating jobs, saying the organisation had much to learn from social programmes pursued by Fidel Castro in Cuba.
"Latin America has all it needs to become a great world power," he said.
"Let's not put any limits on our dreams. Let's make them reality."
Cuban leader Fidel Castro, attending the summit for the first time in three years, said: "This kind of integration has centuries-old enemies."
The summit is primarily about forging economic links across the continent, although Mercosur's members did discuss trade deals with the European Union and other countries such as Pakistan.
Argentina's President Nestor Kirchner said he believed Bolivia and Mexico should be allowed to join the organisation, which he stressed must be a tool for both economic and social development.
But critics of Mercosur claim that it is in danger of losing its free trade focus and becoming more concerned with securing political influence.
"It is hard to see the geographical logic behind Mercosur anymore," said Michael Shifter, from think tank Inter-American Dialogue.
"It is an effort to try to build and consolidate an alternative alliance to US-backed free trade policies."