Thousands of activists marched through Caracas demanding an end to the war in Iraq and shouting slogans against US imperialism Tuesday at the opening of the World Social Forum backed by Venezuelan President Hugo Chavez. The throng paraded along wide avenues chanting "Socialism, yes! Imperialism, no!" to the beat of drums and the trilling of whistles. About 80,000 people signed up to attend the forum, including tens of thousands from outside Venezuela, organizers said. American peace activist Cindy Sheehan, whose son was killed in Iraq, addressed a sea of people from a stage set up alongside a Venezuelan military base, starting a chant of "No to the war!" "We need to bring our troops home immediately," Sheehan told the crowd to sustained 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, California, gained international attention when she set up a protest camp near US President George W. Bush's ranch in Texas last year. Organizers emphasized the event was not intended as a giant rally for Chavez, though the event was backed by funding from his government. A red government truck led the march, and activists stepped aboard to shout slogans, many praising Chavez and shouting "Long live the revolution!" German activist Daniel Poppe, 27, said he believes in Chavez but felt uncomfortable about the truck and the slogans. "I don't like nationalist propaganda," said Poppe, who wore a red Che Guevara T-shirt and said he was considering moving to South America. The marchers included several Palestinians who held an Iraqi flag, and others who stretched out the wings of a giant dove made from white sheets. Several marchers from Syria chanted "Down, down USA!" Others held banners focused on women's rights, indigenous rights or the environment. 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. Those at the social forum, in contrast, traditionally criticize free trade and denounce the evils of capitalism. Chavez, a fierce critic of the US, was expected to address activists on the sidelines, soaking up the spotlight as a leading radical voice of the Latin American left. He has recently strengthened ties with left-leaning leaders across South America. Canopies promoting Chavez's myriad social programs for the poor lined a major avenue in Caracas, while a dozen Venezuelan-made military jeeps were parked on display, along with a tractor made jointly by Iran and Venezuela. Chavez has funneled million of dollars (euros) from booming oil profits into programs for the poor, making him an inspiration for like-minded activists from Canada to Chile. "He's the one who has most influenced the alternative vision in Latin America," said Oliviero Orjuela, a 36-year-old Colombian activist. "Bush embodies the opposite vision - the commercial vision that sees the world as a business." 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.