CARACAS, Venezuela (AP) _ Thousands of activists marched through Venezuela's capital demanding an end to the war in Iraq and shouting slogans against U.S. imperialism at the opening of the World Social Forum backed by Venezuelan President Hugo Chavez.

Throngs of spirited protesters danced to drum beats as they waved banners and chanted ``No to war! Peace is possible!'' Roughly 80,000 people signed up to attend the forum, including tens of thousands from outside Venezuela, organizers said.

``Bush invades countries like Iraq and Afghanistan, takes control of their natural resources and bleeds them dry, that's unacceptable,'' said Candido Gil, a 57-year-old member of Brazil's Communist Party.

Crowds flocked to a stage on the outskirts of a Caracas military base to hear a speech by American peace activist Cindy Sheehan, whose son was killed in Iraq.

``We need to bring our troops home immediately,'' Sheehan told the crowd to a rousing applause. ``We need to hold someone responsible for all the death and destruction in the world. We need to see George Bush and the rest of them tried for crimes against humanity.''

Sheehan, a 48-year-old from Berkeley, Calif., gained international attention when she set up a protest camp near President Bush's ranch in Texas last year.

Many of the activists lauded the leftist movement spreading across Latin America, while others called attention to some of the world's most pressing problems, including air pollution, widespread poverty and racism.

Juergen Schmidt, a 22-year-old German, twisted a few of his dangling dreadlocks while handing out pamphlets titled ``Green Tomorrow'' to fellow activists.

``The human race is threatened by the effects of global warming, and if action isn't taken soon it could be too late,'' said Schmidt, adding that he'd like to learn more about Chavez and ``see what this revolution is all about'' before leaving Venezuela.

Several marchers stretched out giant dove wings made from white sheets while others held banners focusing on indigenous rights, land reform and the evils of capitalism. One man dragged a dirty Coca-Cola banner behind him, yelling ``The global giant will die!''

The World Social Forum was first held in Brazil in 2001 and coincides with the market-friendly World Economic Forum of political and business leaders, which begins Wednesday in Davos, Switzerland.

Nestor Petrola, who traveled to Venezuela by bus from Argentina, said Latin American leaders should look to Chavez as an example to follow.

``He has done more regarding land reform and stopping the privatization of basic services than the others, and that's why he's still in power,'' Petrola said.

Chavez, a leftist former paratrooper and ally of Cuban leader Fidel Castro, was expected to address activists at the closing ceremony of the forum Sunday.

Since first taking office in 1999, Chavez has funneled million of dollars from booming oil profits into programs for the poor, pushed through a new constitution granting new rights to indigenous peoples and established hundreds of state-run cooperatives.

Chavez, who is known as ``El Comandante'' among his supporters, has emerged as one of Latin America's strongest opponents of U.S.-backed free trade proposals and become an inspiration for like-minded activists throughout the region.

This year's social forum is being held in three countries, including a smaller gathering that ended Monday in Mali and another two months from now in Pakistan.

Some 2,000 events _ including seminars, speeches and concerts _ will be held across Caracas this week. Well-known speakers include Uruguayan writer Eduardo Galeano, Argentine Nobel Peace Prize winner Adolfo Perez Esquivel and former French first lady Danielle Mitterrand.

Venezuela deployed 3,500 soldiers and police across Caracas to help keep security.

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Associated Press writers Natalie Obiko Pearson and Ian James contributed to this report.